It may not seem to make sense but most of the universe - mass - can really only account for about 6 percent of what is going on. The rest of it can be under just about any umbrella at this point. Some call it the God of the Gaps, scientists call the unknown mass Dark Matter.
What is it? No one knows, when it comes to the very large and the very small, physics does not have all the answers, but something has made the universe at least 100 billion light years in size even though it's only 13 billion years old. As you know, a light year is the distance light will travel in that time. Since nothing can go faster than the speed of light, it's long been time to think about what Nothing is.
That's dark matter. And whatever is propelling it can be called Dark Energy.