Everything we can see around us, from stars and planets to oceans, trees and people, is made of ordinary matter. Yet ordinary matter accounts for only about 5% of the universe.

The other 95% is something else.

We have mapped galaxies billions of light-years away, detected gravitational waves and photographed the surroundings of black holes. Yet when it comes to what the universe is actually made of, the familiar world of atoms represents only a small fraction of the answer.

About 68% of the universe is dark energy, and its story begins with one of the fundamental discoveries of modern astronomy: the universe is expanding. On the largest scales, galaxies are becoming farther apart as space itself expands.

Scientists once expected gravity to gradually slow that expansion. Instead, observations in the late 1990s revealed something unexpected: the expansion of the universe is accelerating. 

Dark energy is the name scientists give to whatever is causing that acceleration. They still don't know what it is. It could be an intrinsic property of space itself, a dynamic field that changes over time, or evidence that our understanding of gravity is incomplete.

The remaining 27% is dark matter, an invisible form of matter that doesn't emit, absorb or reflect light. Scientists haven't identified what it is, but they can detect its gravitational effects. Galaxies rotate as though they contain far more mass than we can see, and gravity bends light around enormous concentrations of otherwise invisible matter.

In other words, we can observe the effects of roughly 95% of the universe without knowing what that 95% fundamentally is.

Recent observations have made the mystery even more interesting. Data from the Dark Energy Spectroscopic Instrument, or DESI, have produced intriguing evidence that dark energy may change over time rather than remain constant, as the standard model of cosmology assumes. The evidence isn't yet strong enough to claim a discovery, but if it holds up, physicists may need to rethink one of their fundamental assumptions about how the universe works.