what can be found between each galaxy ?

Question

what can be found between each galaxy ?

Answer 1

Energy. Before you think it's a facetious answer, of course, matter and energy are equivalent. The starlight you see travels through intergalactic space, and light is a form of electromagnetic energy.

Also, it has been discovered recently that galaxies (specifically the Andromeda Galaxy, but other discoveries will come) is associated with a huge "halo" of matter, much larger than the galaxy itself and extending a fair fraction of the distance to good old Milky Way galaxy.

Back to the energy. If you believe E=MC^2, then all that light energy is available to (somehow) be converted to matter, in the form of scattered elements. So, instead of being empty, the space between galaxies might (emphasize might) be full of elements here and there. They needn't be close together to have the effect we refer to as dark matter. There is so much room that only a very few per cubic kilometer would suffice.

Answer 2

There is the occasional globular cluster, star, comet, asteroid, or piece of space dust, but mostly there's nothing at all.

Answer 3

Dust, rocks, and my wife's cell phone, wallet, keys, and socks.

Answer 4

ALiiENS!