Galactic dark matter
Galactic dark matter refers to the invisible component of matter that makes up the majority of a galaxy’s mass. It does not emit radiation, does not absorb light, and does not interact with ordinary matter except through gravity. Its presence is inferred from the gravitational effects it exerts on visible matter—in particular, on the orbital velocities of stars in galaxies.
The rotation curve
The most direct evidence for dark matter in galaxies comes from rotation curves. If a galaxy contained only visible matter, stars farther from the center should orbit more slowly (like the outer planets in the solar system). However, observations show that stars on the outskirts of galaxies orbit just as fast as those at the center—or even faster. The only consistent explanation is the presence of a large amount of invisible matter—dark matter—extending far beyond the visible disk.
The halo of dark matter
Galactic dark matter is not concentrated in the galaxy’s visible disk. It forms a vast spherical halo that envelops the entire galaxy. For the Milky Way, this halo likely extends to several hundred thousand light-years and accounts for about 85% of the galaxy’s total mass— well beyond what the stars, gas, and dust combined account for.
Unknown Nature
Despite decades of research, the nature of dark matter remains a mystery. The most widely studied theoretical candidates include weakly interacting massive particles (WIMPs), axions, sterile neutrinos, or primordial black holes. Direct and indirect detection experiments have been attempting to identify these particles for many years, with no confirmed success to date.
Concrete example
The Draco spheroidal dwarf galaxy, a satellite of the Milky Way, is one of the objects known to have the highest proportion of dark matter. Its dynamic mass (derived from the velocities of its stars) is about 250 times greater than the mass of its visible stars. It is an ideal laboratory for studying the properties of dark matter in a relatively simple environment.
Frequently Asked Questions
Is galactic dark matter the same as cosmological dark matter?
Yes. Dark matter observed at thegalactic scale (via rotation curves) and on the cosmological scale (via the cosmic microwave background and large-scale structure) is the same component. Both lines of evidence point to the same properties and the same abundance.
Why not modify gravity rather than invoke dark matter?
Modified gravity theories such as MOND (Modified Newtonian Dynamics) attempt to explain rotation curves without invoking dark matter. They work reasonably well for isolated galaxies, but fail to explain the behavior of galaxy clusters and cosmological observations. Dark matter remains the hypothesis best supported by the body of observations.
Could dark matter be ordinary black holes?
Ordinary stellar-mass black holes cannot account for all of the dark matter—observations of gravitational microlensing effects impose strict constraints on their abundance. Primordial black holes (formed in the early universe) remain a partial theoretical candidate, but observational constraints also limit their possible contribution.