Physicists have a pretty good ways of measuring the total mass present in the universe. They also have some pretty accurate ways of measuring the total mass we can observe, but here's the catch-the two numbers don't match up.
In fact, the amount of total mass in the universe is vastly greater than the total mass we can actually account for. Physicists were forced to come up with an explanation for this, and the leading theory right now involves dark matter, a mysterious substance that emits no light and accounts for approximately 95% of the mass in the universe. While it hasn't been formally proved to exist (because we can't see it), dark matter is supported by a ton of evidence, and has to exist in some for or another in order to explain the univers.