We know that spiral galaxies contain dark matter because the gas in the disk rotates with a velocity that decreases in a way that is proportional to r^-1/2.

Question

Quizplus · Accepted Answer

The rotation curves of spiral galaxies remain flat or even rise at large radii, rather than decreasing as expected if only visible matter were present. This discrepancy suggests the presence of dark matter.

We Know That Spiral Galaxies Contain Dark Matter Because the Gas