Observations of distant galaxies indicate that the Universe

The stars and gas in the Galaxy rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Galaxy does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s.[106] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the galaxy the orbit speeds are too low while beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.
If the Galaxy contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[22] Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve
In addition to the stellar halo, the Chandra X-ray Observatory, XMM-Newton, and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light years, much further than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the galaxy itself.[92][93][94] The temperature of this halo gas is between 1 million and 2.5 million kelvin, a few hundred times hotter than the surface of the sun.[95]
Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.[96] If the finding that the mass of the halo is comparable to the mass of the galaxy is confirmed, it could be the identity of the missing baryons around the Milky Way