Galaxies were once thought of as lonely islands in the universe: clumps of matter floating through otherwise empty space. We now know they are surrounded by a much larger, yet nearly invisible cloud of dust and gas. Astronomers call it the circumgalactic medium, or CGM. The CGM acts as a giant recycling plant, absorbing matter ejected by the galaxy and later pushing it right back in.
Source:
https://phys.org/news/2019-10-stars-pollute-galaxies-recycle.html