The expansion of the universe is due to dark energy, a mysterious form of energy that surrounds us and permeates the entire cosmos. But its exact characteristics remain unknown, with most of what we know about dark energy coming from its effect on galaxies.

Now, a new study from scientists at the University of Alabama at Huntsville, published in the Monthly Notices of The Royal Astronomical Society, has claimed that dark energy has not changed over billions of years. Astronomers used the size of galaxy clusters as “cosmic rulers,” and compared nearby clusters to far away ones. If the rate of expansion across cosmic time has changed, their relative size must have changed too, but the scientists did not observe this phenomenon.

This discovery was possible thanks to NASA’s X-ray Chandra telescope. Astronomers took advantage of the fact that galaxy clusters appear very similar when seen in X-rays, selecting 300 different clusters with a distance ranging from 760 million to 8.7 billion light-years.

The team likened this approach to a “Russian doll,” because they imagined fitting all these galaxy clusters into each other. If the expected cosmological parameters change along the way, the clusters don’t stack up anymore and the Russian dolls don’t fit into each other.

This latest study brings confirmation to the idea that dark energy is a cosmological constant, often seen as the energy that generates from empty space itself. The need for dark energy comes from the fact that the universe is not simply expanding, but it is undergoing an accelerated expansion; the further a galaxy is from us, the faster it appears to move away.

The cosmological constant theory is not the only explanation for dark energy, though, with several other mechanisms proposed to explain what we observe. Findings like the recent discrepancy in expansion rate indicate that the mystery is far from settled, and only more observations might get us closer to understanding the dark side of the cosmos.