Giant galaxies such as the Milky Way and Andromeda consist mostly of exotic dark matter. But even our galaxy's ordinary material presents a puzzle since most of it is missing and remains undiscovered by scientists. Now, however, by watching a galaxy plow through the Milky Way's outskirts, astronomers have estimated the amount of gas surrounding our galaxy's bright disk, finding that this material outweighs all of the interstellar gas and dust in our part of the Milky Way.

Measurements of the cosmic microwave background—the big bang's afterglow—indicate that one sixth of all matter in the universe is ordinary, or baryonic, containing protons and neutrons (or "baryons" in the parlance of physicists), just as stars, planets and people do. Based on the motion of distant objects orbiting the Milky Way, astronomers estimate that our galaxy is roughly a trillion times as massive as the sun. If five sixths of this material is dark matter, then this exotic substance makes up 830 billion solar masses of our galaxy; baryonic matter should account for the remaining 170 billion.

The trouble is, all of our galaxy’s known stars and interstellar matter add up to only about 60 billion solar masses: 50 billion in stars and 10 billion in interstellar gas and dust. (The Milky Way has more than 100 billion stars, but most are smaller than the sun.) That leaves a whopping 110 billion solar masses of ordinary material unaccounted for. If the Milky Way is even more massive than currently estimated, this missing baryon problem gets worse—and the same conundrum afflicts other giant galaxies as well.

Where are the missing baryons? Perhaps in a diffuse gaseous halo around the Milky Way. X-ray satellites have detected oxygen atoms in our galaxy that have lost most of their eight electrons, a sign they inhabit gas that is millions of degrees hot—far hotter than the surface of the sun. But since we don’t know how far these fried oxygen atoms are from us, we can’t accurately gauge the size of this component of the galaxy. If they're fairly close to the disk, then this so-called circumgalactic medium isn't extensive and therefore doesn't amount to much. But if they're far away, spread throughout a gargantuan halo, this gaseous material could outweigh all of the galaxy's stars, providing fuel for star formation for billions of years to come.

Fortunately for astronomers, the Milky Way is so mighty that it rules a retinue of smaller galaxies that revolve around it just as moons orbit a planet. The most splendiferous satellite galaxy is the Large Magellanic Cloud, shining 160,000 light-years from Earth. Like all the other galactic satellites, this one moves around the Milky Way, but unlike most of its peers, it abounds with gas, which gets stripped as it rams into the halo's own gas. The amount of gas lost depends on the speed at which our neighbor moves and how dense the halo gas is. And that density can yield a mass estimate for the halo's gas.

Recently, the Hubble Space Telescope measured the galaxy's speed. This allowed astronomers Munier Salem of Columbia University, Gurtina Besla of the University of Arizona and their colleagues to study the stripped gas and estimate that the gas density in the Milky Way's halo near the Large Magellanic Cloud is 0.0001 atoms per cubic centimeter. That's not much—only about 10,000 times more tenuous than the interstellar gas in the Milky Way's disk—but the halo covers a lot of real estate. In research submitted for publication in The Astrophysical Journal, the astronomers assume that the gas density declines with distance from the Milky Way's center, and calculate that the gas adds up to 26 billion solar masses, or close to half the amount in all of the Milky Way's stars. Matthew Miller, a graduate student at the University of Michigan who is completing his dissertation on the circumgalactic medium, says this number corresponds with previous estimates but is based on a more direct measurement of the density.

Still, the newly calculated halo gas mass makes up just 15 percent of the Milky Way's expected baryonic content. Besla says the true quantity of the halo gas is probably greater because its density may decline less with distance than standard models assume. Miller suspects the missing baryons may be absent from the Milky Way altogether, having never fallen into our galaxy with the dark matter, in which case they are drifting in the vast space between giant galaxies.

Besla predicts that future work may yield a better measurement. Another gas-rich galaxy—the Small Magellanic Cloud, 200,000 light-years from Earth—orbits the Large Magellanic Cloud. Their dance has spilled gas into a stream more than half a million light-years long. Most of this Magellanic Stream extends beyond the Large Magellanic Cloud and thus should probe the halo's gas density elsewhere, Besla says, further constraining the mass of the circumgalactic medium.

Indeed, astronomers on Earth are lucky: They inhabit one of the few giant galaxies boasting two nearby gas-rich satellite galaxies. "It's amazing how much information this system provides us," Besla says. In contrast, all satellites orbiting a more typical giant galaxy have run out of gas, and any astronomers there may look upon their peers in the Milky Way with quiet envy.