Just how big Black Holes can get.
Massive enough to affect galaxies in a cluster.

And possible effects of Dark Matter in their creation.

https://www.msn.com/en-ca/weather/topstories/monster-black-hole-is-caught-feeding-off-galaxies-trapped-in-its-spider-s-web/ar-BB19LywO?ocid=msedgntp

2 hours ago, MigL said:

Massive enough to affect galaxies in a cluster.

Yes. You surely talk about the Sphere of influence.

Not all matter is drawn into the hole. It forms the so-called accretion disk - a bright cloud of gas and other substances. It is because of this disk that the galaxies glow so brightly - it glows brighter than 300 trillion suns. It is also the cause of turbulence. It is not yet completely clear whether all the gas pushed outward will leave the galaxy, or whether it will be swallowed up by a black hole.