There is a number that cosmologists rarely lead with, because it is quietly devastating. It is 16 billion light-years. Beyond that distance, every galaxy (every trillion stars, every possible civilization, every structure in the universe) has already sent us its last message. We simply have not received it yet. That number is the dark energy cosmic event horizon, the boundary beyond which dark energy has already severed our connection to the rest of the universe.
Dark energy is winning. It has been winning since the universe was roughly 7 billion years old. And the margin of victory grows every second.
What Dark Energy Actually Is (And Why That Is the Honest Answer)
Here is what cosmologists will tell you about dark energy, and why it matters that they say it this way: we do not know what it is.
What we know is what it does. It causes the expansion of the universe to accelerate. It behaves as though empty space has an intrinsic energy, a property Einstein called the cosmological constant, which he introduced in 1917 and famously abandoned, only for the universe to vindicate him seventy years after his death.
Dark energy accounts for approximately 68% of the total energy density of the universeDark matter accounts for another 27%. Everything you have ever seen, touched, measured, or theorized about (every atom, every photon, every black hole) makes up roughly 5%.
The remaining 95% of the universe is, in the most literal sense, unknown.
What makes dark energy particularly strange is that it does not dilute. Ordinary matter spreads thinner as the universe expands. Dark energy does not. As space grows, the total amount of dark energy grows with it. This is why the expansion is not merely continuing but accelerating: the more space there is, the more dark energy there is, the faster space expands, the more space there is.
It is a runaway process. And it has been running for 7 billion years. This acceleration began to dominate the universe’s expansion roughly 7 billion years ago, when the density of matter, diluted by expansion, fell below the constant density of dark energy, causing the universe’s expansion to shift from deceleration to acceleration.
The Discovery That Changed Everything
In 1998, two independent research teams were racing to measure how fast the universe’s expansion was slowing down. Everyone assumed it was slowing; gravity acts on mass, mass fills the universe, the expansion should be decelerating.
Both teams found the opposite.
The universe was not slowing. It was speeding up. The evidence came from Type Ia supernovae, stellar explosions that always detonate at nearly the same intrinsic brightness, making them reliable distance markers across cosmic scales. The supernovae were dimmer than expected, meaning they were farther away than a decelerating universe would place them.
Saul Perlmutter, Brian Schmidt, and Adam Riess shared the 2011 Nobel Prize in Physics for the discovery. In their acceptance remarks, all three noted the same thing: the result had not been what anyone was looking for.
The universe had been hiding an accelerant.
How the Dark Energy Cosmic Event Horizon Cuts Us Off
Here is the mechanism, stated plainly.
Every galaxy beyond our Local Group (the small cluster of about 50 galaxies gravitationally bound to the Milky Way) is receding from us. The farther away a galaxy is, the faster it recedes. At the cosmic event horizon, approximately 16 billion light-years away, galaxies are receding at exactly the speed of light. Beyond that boundary, they recede faster. (For a deeper look at the nature of this horizon, see our article on how far the cosmic event horizon is).
This is not movement through space. The galaxies are not going anywhere. Space itself is expanding, and the expansion of enough space between two objects adds up to a recession speed that can exceed the speed of light without violating relativity. No information, no signal, no mass is traveling faster than light. Space is simply creating more of itself.
The consequence: any photon emitted by a galaxy beyond the event horizon today is swimming upstream against an expansion that outpaces it. That photon will never arrive. The galaxy has, for all cosmologically practical purposes, sent its last letter.
We will continue to receive light from those galaxies for billions of years, light emitted in the past, still in transit. But those galaxies are already gone from our communicable universe. The lights are still on. Nobody can answer the door. In fact, the last photons from these galaxies will trickle in over eons, becoming infinitely redshifted and faint, a process of “fading to black” that completes their departure from our observable reality.
A Note on Distance: The 16-billion-light-year horizon is the proper distance now—how far away the object is at this moment. The light we see from such an object has been traveling for nearly the universe’s 13.8-billion-year age, but the expansion of space has stretched the distance between us during that journey. This distinction between light-travel distance and current proper distance is key to understanding cosmic horizons.
The Scale of What We Are Losing
The observable universe contains an estimated 2 trillion galaxiesThe number within our cosmic event horizon (the number we could in principle ever reach, signal, or receive signals from in the future) is far smaller, and shrinking.
The Virgo Supercluster alone, containing the Milky Way and thousands of neighboring galaxies, sits well within our horizon. The broader Laniakea Supercluster, a structure 500 million light-years across, is largely accessible. But beyond that, the losses mount quickly.
Cosmologist Lawrence Krauss has described the long-term view in stark terms: in 100 billion years, when the universe is roughly 7 times its current age, the night sky as seen from any galaxy will be essentially dark. The expansion will have carried all other galaxies beyond the event horizon. The cosmic microwave background (the faint afterglow of the Big Bang) will have redshifted below detectability. A civilization born in that era will have no observational evidence that the Big Bang occurred, no evidence that other galaxies exist, and no ability to measure the expansion of the universe because there will be nothing left to measure it against.
They will inherit a universe that appears, by all observation, to be static, eternal, and alone.
Why the Milky Way Is Safe (For Now)
Not everything is lost. Gravity still wins locally. The Milky Way and Andromeda are on a collision course; they will merge in approximately 4.5 billion years into a single giant elliptical galaxy astronomers have already named Milkomeda. The entire Local Group will remain gravitationally bound, drawing tighter together even as the broader universe expands away from it.
The same is true for galaxy clusters. Within a gravitationally bound structure, dark energy cannot overcome local gravity. The acceleration operates on the largest scales: the stretching of the web between clusters, not the clusters themselves.
What dark energy is dismantling is not structure. It is connection. The threads of the cosmic web that link superclusters are slowly fraying. The isolated islands of gravity (galaxy groups and clusters) will remain intact, but they will drift apart into an expanding void with no bridges between them.
What the James Webb Space Telescope Is Teaching Us
The James Webb Space Telescope has added a new layer of complexity to the dark energy picture. Its observations of the very early universe (galaxies formed within the first few hundred million years after the Big Bang) have revealed structures that challenge existing models of how quickly the universe could have organized itself.
These “impossible galaxies,” as some astronomers informally call them, are more massive and more structured than current models of early galaxy formation and dark matter halo assembly predict they should be. Whether they represent a flaw in the models, new physics, or systematic measurement error is an active area of debate. This research is distinct from the study of dark energy’s accelerating effects, but it underscores a broader truth: our understanding of cosmic evolution, from the first galaxies to the fate of the universe, is still being written.
What is not in dispute is the basic architecture: the universe expanded slowly at first, was pulled by dark energy into acceleration, and is now in a runaway phase that will, over cosmological time, leave the observable universe populated by nothing but a handful of gravitationally bound island universes, embedded in an otherwise featureless, expanding dark.
The Equation of State
Dark energy is characterized by a quantity called its equation of state, written as wFor a cosmological constant (the simplest form of dark energy, and the one most consistent with current data) w = −1. While the cosmological constant is the leading model, other theoretical explanations exist, such as dynamic fields called quintessence or modifications to Einstein’s theory of General Relativity.
If w is slightly less than −1, the dark energy grows stronger over time, eventually tearing apart not just the connections between galaxies but the galaxies themselves, then solar systems, then planets, then atoms. This scenario is called the Big Rip. Current observations from missions like Planck are consistent with w = −1 but cannot rule out the Big Rip entirely.
If w is slightly greater than −1, dark energy might eventually weaken. Some models allow for a future in which the acceleration slows or reverses, though there is no observational evidence for this.
We are, in the most technical sense, riding a cosmological constant whose nature we do not understand, whose future behavior we cannot predict, and whose effects are already severing the universe’s connections one galaxy at a time.
What is dark energy?
Dark energy is the name given to the unknown force or property of spacetime causing the universe’s expansion to accelerate. It accounts for approximately 68% of the total energy density of the universe. It behaves as though empty space has intrinsic energy (consistent with Einstein’s cosmological constant), but its fundamental nature remains one of the deepest unsolved problems in physics.
How do we know dark energy exists?
The strongest evidence comes from observations of Type Ia supernovae, which serve as standardizable distance markers across cosmic scales. In 1998, two independent teams found that distant supernovae were dimmer (and therefore farther) than a decelerating universe would place them. The universe was accelerating. Combined with measurements of the cosmic microwave background and large-scale structure surveys, dark energy is the only model consistent with all the data.
Will dark energy eventually destroy the Milky Way?
Not under the standard cosmological constant model. Dark energy operates on the largest scales and cannot overcome gravity within a gravitationally bound structure. The Milky Way, Local Group, and eventually Milkomeda will remain intact. What dark energy destroys is not local structure but the connections between galaxy clusters: the cosmic web that links the universe’s islands of matter.
What is the Big Rip?
A hypothetical future scenario in which dark energy strengthens over time rather than remaining constant. If the equation-of-state parameter w is less than −1, dark energy eventually overcomes not just intergalactic space but local gravity, tearing apart galaxy clusters, then individual galaxies, then solar systems, then planets, then subatomic particles, all within a finite time. Current observations are consistent with w = −1 (no Big Rip) but cannot rule it out.
How far away is the cosmic event horizon?
The future cosmic event horizon (the maximum distance from which light emitted today could ever reach us) is approximately 16 billion light-years, given the current rate of acceleration. Galaxies beyond this boundary are already cosmologically unreachable, not because of distance alone, but because the expansion of space between us and them outpaces the speed of light.
The dark energy cosmic event horizon relationship is not just theoretical: it sets a hard limit on how much of the universe will ever be accessible to us.
Sources
- Perlmutter, S. et al. (1999). Measurements of Omega and Lambda from 42 High-Redshift Supernovae. The Astrophysical Journal, 517(2), 565–586.
- Riess, A.G. et al. (1998). Observational Evidence from Supernovae for an Accelerating Universe. The Astronomical Journal, 116(3), 1009–1038.
- Planck Collaboration (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics, 641, A6.
- Krauss, L.M. & Scherrer, R.J. (2007). The Return of a Static Universe and the End of Cosmology. General Relativity and Gravitation, 39(10), 1545–1550.
- Caldwell, R.R., Kamionkowski, M. & Weinberg, N.N. (2003). Phantom Energy: Dark Energy with w < −1 and the Big Rip. Physical Review Letters, 91(7), 071301.
- Labbé, I. et al. (2023). A population of red candidate massive galaxies ~600 Myr after the Big Bang. Nature, 616, 266–269.