Starship Asterisk*

Dark Matter seems not to interact with anything we can detect, or to reveal itself except by gravitational effects.
But is it thought to interact with itself? What might happen when galactic haloes meet - for example our galaxy and M31 in Andromeda? Two pennies, placed about twenty diameters apart, give a rough idea of the current separation, but might their Dark Matter haloes already be interacting?

starbrush wrote:Dark Matter seems not to interact with anything we can detect, or to reveal itself except by gravitational effects.
But is it thought to interact with itself? What might happen when galactic haloes meet - for example our galaxy and M31 in Andromeda? Two pennies, placed about twenty diameters apart, give a rough idea of the current separation, but might their Dark Matter haloes already be interacting?

There isn't much (or any) evidence that dark matter interacts with itself except through gravitation. When galaxies collide, their dark matter halos do interact, which has been observed.

The halos around the Milky Way and Andromeda are not large enough to have come into contact yet, or for their to be any significant tidal interaction.

starbrush wrote:
What might happen when galactic haloes meet?

They will tidally elongate into prolate spheroids.

Chris Peterson wrote: There isn't much (or any) evidence that dark matter interacts with itself except through gravitation. When galaxies collide, their dark matter halos do interact, which has been observed.

The halos around the Milky Way and Andromeda are not large enough to have come into contact yet, or for their to be any significant tidal interaction.

Confirming Chris' point:

the presence and distribution [of dark energy] is found indirectly through its gravitational effects. The gravity from both dark and luminous matter warps space, bending and distorting light from galaxies and clusters behind it like a giant magnifying glass. Astronomers can use this effect, called gravitational lensing, to infer the presence of dark matter in massive galaxy clusters.

(As I think we all know, dark matter itself is not visible.)

Subsequent to the research mentioned in Chris' link ["observed"], a new observation of Abell 520 from another team of astronomers using a different Hubble camera finds that the core does not appear to be over-dense in dark matter after all. The study findings were published in The Astrophysical Journal.

Read more at: https://phys.org/news/2012-11-dark-core ... y.html#jCp