Weighing a cloud of hot gas the size of a million galaxies is no easy task. Astronomers observe how this gas bends light passing by, and from that bending they calculate its mass. But the gas in galaxy clusters is not static: it churns like a boiling pot, and some of the pressure goes into turbulence rather than holding the gas against collapse. As a result, the cluster's mass comes out underestimated, and all dark matter calculations start to drift.
To catch this error, scientists propose combining three different observations of the same patch of sky. First, how light from distant galaxies is distorted on its way to us, like the bottom of a pool seen through ripples on the water. Second, how the galaxies themselves are distributed in that patch. Third, the faint glow of hot gas left over from the Big Bang, slightly warmed as it passes through the cluster. Each measurement alone is imprecise, but together they yield a correction accurate to one percent.
Future sky surveys, such as the Vera Rubin Observatory and the CSST space telescope, will be able to make these measurements. Within a few years, the fraction of dark matter in the universe will be known to better than one percent, and finally it will become clear how fast the universe is expanding.
A galaxy cluster can weigh as much as a million billion Suns, but the hot gas in it takes up ten times more space than the galaxies themselves.