The universe is expanding faster because something is pushing it apart, overpowering gravity on the largest scales. That something is called dark energy, a mysterious repulsive force that makes up roughly 68% of the total energy content of the universe. Since its discovery in 1998, dark energy has become one of the biggest open questions in physics.
What We Know About Dark Energy
The universe contains three main ingredients: ordinary matter (about 5%), dark matter (about 27%), and dark energy (about 68%). Ordinary matter is everything you can see and touch, from stars to oceans to your own body. Dark matter is invisible but exerts gravitational pull, holding galaxies together. Dark energy does the opposite. It acts as a kind of repulsive gravity, pushing space itself apart and causing distant galaxies to fly away from each other at ever-increasing speeds.
The leading explanation is that dark energy is the energy of empty space itself. Every cubic meter of vacuum contains a small but constant amount of energy, and as the universe expands, new space appears with its own built-in energy. This means the total amount of dark energy grows as the universe gets bigger, while gravity’s pull between objects weakens as they move farther apart. Over billions of years, dark energy wins.
Einstein actually predicted something like this in 1917. When he applied his general theory of relativity to the structure of the cosmos, he found that the equations allowed for a repulsive, space-filling energy. He called it the cosmological constant, or lambda. He originally introduced it to keep the universe from collapsing under its own gravity, then abandoned it when astronomers discovered the universe was expanding. Decades later, it turned out he may have been right after all, just for the wrong reasons.
How Scientists Discovered the Acceleration
Until the late 1990s, most physicists assumed the expansion of the universe was gradually slowing down. Gravity should be pulling everything back together, like a ball thrown upward losing speed. Two independent research teams set out to measure exactly how much the expansion was decelerating: the Supernova Cosmology Project and the High-Z Supernova Team.
Both teams used a specific type of exploding star called a Type Ia supernova as a cosmic measuring tool. These supernovae all reach roughly the same peak brightness, which means astronomers can calculate how far away they are by measuring how dim they appear. By comparing that distance to how fast the light from each supernova has been stretched by the expanding universe, you can figure out whether the expansion is speeding up or slowing down.
The answer shocked everyone. The distant supernovae weren’t brighter than expected (which would mean a slowing expansion). They were dimmer. That meant they were farther away than they should have been, which could only happen if the expansion of the universe had been accelerating while their light was traveling toward Earth. In 1998, both teams reported this finding independently. The discovery earned three of the lead researchers the Nobel Prize in Physics in 2011.
The Evidence Has Only Gotten Stronger
Since 1998, multiple independent lines of evidence have confirmed that cosmic acceleration is real. The Dark Energy Survey, using roughly 1,500 high-redshift supernovae gathered over five years, now confirms acceleration at greater than 5-sigma confidence. In statistical terms, that means there’s less than a 1 in 3.5 million chance the result is a fluke.
Another powerful tool is baryon acoustic oscillations, a pattern imprinted in the distribution of galaxies that acts as a cosmic ruler. In the early universe, sound waves rippled through the hot plasma of matter and radiation, creating regions of slightly higher density spaced about 500 million light-years apart. That spacing is still visible today in how galaxies cluster together. By measuring this fixed ruler at different points in cosmic history, scientists can map how the expansion rate has changed over time. The results line up: the expansion is accelerating.
Observations of the cosmic microwave background, the faint afterglow of the Big Bang, provide yet another independent check. Data from the Planck satellite paints a picture of the universe’s composition that requires dark energy to make the math work. All three methods, supernovae, galaxy clustering, and the cosmic microwave background, point to the same answer.
The Hubble Tension: A Crack in the Picture
While scientists agree the universe is accelerating, they don’t fully agree on how fast it’s expanding right now. This disagreement is called the Hubble tension, and it could be a clue that something deeper is going on.
The expansion rate is measured by a number called the Hubble constant. When the Planck satellite calculates this value using observations of the early universe and the standard model of cosmology, it gets about 67.4 kilometers per second per megaparsec. When the SH0ES team measures it directly using nearby supernovae and pulsating stars, they get 73.04, roughly 9% higher. The gap between these two numbers now sits at a 5-sigma level of statistical significance, meaning it’s very unlikely to be a measurement error.
If both measurements are correct, something about our understanding of the universe is incomplete. The discrepancy could point to new physics: perhaps dark energy isn’t perfectly constant, or perhaps there’s an unknown component in the early universe that made it expand slightly faster than our models predict. No one has a definitive answer yet.
Is Dark Energy Constant, or Is It Changing?
The simplest version of dark energy, Einstein’s cosmological constant, stays the same strength everywhere and at all times. But that’s not the only possibility. An alternative idea called quintessence proposes that dark energy is actually a dynamic field, one that can change in strength as the universe evolves. Think of the cosmological constant as a fixed setting on a thermostat, while quintessence is more like a dial that slowly turns.
Current observations can’t definitively tell these two ideas apart. The Dark Energy Survey’s latest results find that dark energy is consistent with a cosmological constant to within about 2 sigma, which means there’s still room for it to be something slightly different. Upcoming missions, including the Euclid space telescope and the Vera C. Rubin Observatory, are designed specifically to push that measurement further and determine whether dark energy has been changing over cosmic time.
There’s also a more radical possibility: cosmic acceleration might not come from a mysterious energy at all. Instead, it could signal that Einstein’s general relativity breaks down at the largest scales and needs to be replaced with a more complete theory of gravity. This idea is harder to test, but it remains on the table.
What Acceleration Means for the Future
If dark energy remains constant (or grows stronger), the long-term fate of the universe is a slow, cold death sometimes called the Big Freeze. The expansion will never reverse. It will only accelerate.
Within a couple trillion years, the universe will have expanded so much that no galaxies beyond our own local group will be visible. The light from distant galaxies will be stretched beyond detection, and any civilization alive at that point will see nothing but darkness beyond its own neighborhood. About 100 trillion years from now, all star formation will stop as galaxies run out of the gas needed to make new stars. In the eons that follow, stellar remnants will decay, and all remaining matter will end up inside black holes.
Even the black holes won’t last forever. Over timescales almost impossible to comprehend, a googol years into the future (that’s a 1 followed by 100 zeroes), even supermassive black holes will evaporate through a slow quantum process called Hawking radiation. What’s left is a universe with no structure, no matter, and no usable energy, just an incomprehensibly vast expanse hovering barely above absolute zero. This final state, called the Dark Era, will last far longer than everything that came before it.