Virial clouds and rotational asymmetry in galactic haloes

In 1995, it was suggested that some of the baryonic dark matter in galaxies may be in the form of molecular hydrogen clouds, and a mechanism for observing them had been given. In the same year, a novel method of seeing the clouds was proposed, that is to look for a temperature asymmetry in the cosmic microwave background towards the M31 galaxy, due to a “Doppler effect” induced by the M31 halo rotation. This temperature asymmetry has since been seen and confirmed in M31 and other galaxies, and used to study the rotation of galactic haloes and map their dynamics. It had been questioned whether such clouds could actually exist, and in response, the clouds were modeled and shown to be possible. It then becomes necessary to trace the evolution of those clouds from their formation to the modern day. Here, the development of the ideas is reviewed.