The Andromeda galaxy is our nearest large neighbor, and like the Milky Way, it has an invisible gaseous shroud. This shroud of extremely hot, rarefied gas extends for hundreds of thousands of light-years. Seeing it directly is nearly impossible — the gas is too faint. But when light from a distant quasar passes through this halo, the gas leaves its "fingerprints" in the light's spectrum — dark absorption lines.
Astronomers used 15–18 quasars whose light pierces Andromeda's halo at various distances from the center. By comparing how much gas "ate" light along close-in and far-out sightlines, they noticed that at the very center of the halo, at a distance of about 300–350 thousand light-years, there is slightly more gas than would be expected from the Milky Way alone. This excess is likely the hot envelope of Andromeda itself.
Estimates of this gas's mass depend strongly on where the halo ends. If it is assumed to end at the "virial radius" (the approximate boundary where gas is still gravitationally bound to the galaxy), then the gas mass could exceed the entire "missing" baryon budget — the ordinary matter that astronomers have long been searching for. If the halo extends farther, to 400–500 thousand light-years, the mass drops to a few hundred billion solar masses. For now, there is not enough data to choose between scenarios.
To settle the matter, longer observations of the same quasars and new sightlines passing even closer to Andromeda's center are needed. Then it will become clear how much hot gas the neighboring galaxy hides and whether this helps solve the mystery of the missing matter in the universe.
Andromeda's hot halo is so rarefied that if you flew through it in a spaceship, you would not feel the slightest resistance, but the total mass of this "nothing" could exceed the mass of all the stars in the galaxy.