Astronomers know that dark matter is present in galactic halos because the speeds of orbiting stars is constant far from the center of the galaxy.
The speed of stars is in the range of 1 kpc to 16 kpc from the galactic centre.
These stars move approximately on circular orbits about the galactic centre along with small peculiar velocities.
For most of the stars, the velocities range between 200 km/s to 250 km/s.
The speed of the stars orbiting stars remains constant far from the center of the galaxy.
Thus, Astronomers know that dark matter is present in galactic halos because the speeds of orbiting stars is constant far from the center of the galaxy.
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