Why is the expansion of the universe speeding up?

For a long time, astronomers expected gravity to slow the universe’s expansion. Galaxies were moving apart, but their mutual pull should have slowed the growth of distance, like a brake. That is only an analogy: gravity does not press on a galaxy moving along a road. In the late 1990s, two teams studied distant exploding stars and found the opposite story: the expansion had begun to speed up. Later measurements strengthened that result.

The stars were Type Ia supernovae. After careful calibration, their brightness tells us roughly how far away they are. Their light also tells us how much the universe expanded while it traveled. The distant supernovae looked dimmer than expected for their redshifts, meaning they were farther away than a slowing-only history predicted. It was a clue about how the expansion rate changed over billions of years, not a movie of galaxies visibly accelerating during one night.

What causes it? We call the missing explanation dark energy. That name does useful work: it points to an effect that must be accounted for in our best models. But it does not tell us what the effect physically is. The simplest standard model uses a cosmological constant, a term that acts like a fixed property of space. Other models let the effect vary with time or change how gravity behaves across immense distances.

There is a fresh reason to keep the question open. In March 2025 the DESI collaboration reported that, when its galaxy map is combined with other observations, a changing dark-energy model may fit better than the constant one. DESI’s data alone remain consistent with the standard constant model. The combined hint did not reach the usual discovery threshold. Cosmic acceleration is well supported; the identity and history of its cause remain active research questions.