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.
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