Much of the mass of our galaxy appears to be in the form of "dark matter" of unknown composition. at present, this matter can be detected only because:

At present, this matter can be detected only because " its gravitational pull affects orbital motions of matter in the galaxy"

Explanation:

A hypothetical galaxy with very few starts, although named so as they have no visible stars. But if they are fulfilled with large amounts of gas they can be observable and understood them as "Dark galaxy".

While they vary from intergalactic gas clouds induced by galactic tidal interactions, because such gas clouds have absence of dark matter therefore technically they are not covered as galaxies.

The Milky Way's much of the mass observed to be dark matter, which is an invisible type and unknown matter, have interaction with ordinary matter but gravitationally. Astronomers estimated that the Milky Way galaxy mass is more than twice as much as previous studies had estimated, this directs that 90 % of the galaxy mass is dark matter as reported in March 2019.

answer; equivalent to a one(1) horsepower motor;