Two of the three galaxies are forming new stars at a high rate.

For a start, dark matter could not form stars.

For example, dusty galaxies just starting to form new stars.

More mature galaxies typically form stars at a rate of only a few a year.

Some will dissipate before they can collapse to form stars.

There are even a few examples of galaxies that have only started forming stars in the recent past.

Dark matter can't form stars, because it can't cool down fast enough.

This is because these two elements (and sometimes lithium) are the only ones that form outside stars.

Why haven't they warmed, dissipated or collapsed to form stars?

The result of all this violence is that galaxies tend to have little gas available to form new stars after they merge.