Is there anything between galaxies?

Two things: dark matter and dark energy.
When I first started this page, I wrote a series of posts related to dark matter, and as I said back then, we call it this way not because of a specific color, but because we cannot see it. Dark matter should in fact be called invisible matter as it doesn’t emit any electromagnetic radiation or, to put it in simpler terms, it does not interact with light. There is only one pseudo-interaction between dark matter and light: it can bend it due to gravitational force. It is for this reason (among some others) we experience gravitational lensing when we look at distant galaxies and see them distorted and curved instead of seeing them in their original elliptical or spiral form (wrote a post back in April about this).
Back to Navindra’s question! Let’s take our galaxy, the Milky Way. The supermassive black hole at the center, Sagittarius A*, is supposedly gravitationally powerful enough to keep all the stars, nebulas, planets etc. in the two galactic spirals and everything in between in place and rotating around it. Well… it kind of isn’t. Astronomers have been building virtual models for the past decade using the data we have on supermassive black holes dominating the center of galaxies and the result is always the same. A supermassive black hole alone is never powerful enough to keep the galaxy together, so there must be something else. Some sort of matter that is keeping everything in place in the Universe. 

It is believed that the Universe is made up of 4% normal matter, 22% dark matter and 76% dark energy. We know something about dark matter (that it has mass, gravitational pull etc.) but we know close to 0 about what dark energy is and where it comes from. Without dark matter, every galaxy in the universe would tear apart, as the supermassive black holes at the center of galaxies would be to faint to keep everything in place. Lately there have been a series of books written on the subject and the best (in my opinion) is Katherine Freese’s The Cosmic Cocktail: Three Parts Dark Matter. – Roman Alexander

(The question was originally asked by Navindra Roopnarine from Trinidad and Tobago)