The astronomer Edwin Hubble discovered, in 1929, that the universe expands. This is the origin of the "big bang" theory. However, 70 years later, in 1999, Saul Perlmutter, Brian P. Schmidt and Adam G. Riess (and their collaborators) discovered that not only the universe expands, but its expansion rate is, in fact, accelerating. This came as a big surprise, since on very large scale, one expects the only relevant force to be gravity; and gravity only attracts, but does not repel. Thus, there must be something else, different than all known forces in nature, that causes the universe expansion to accelerate. Nobody knows the nature of this mysterious force. It is known, though, that the energy stored in it is huge, and accounts for nearly 70% of all the energy (including mass, since E=m c2, and including "dark mass") of the entire universe.
There have been, of course, several attempts to understand its origin. One way of explaining it is by introducing Einstein's "cosmological constant", which basically states that the vacuum itself has energy. This is aligned with the ideas of quantum mechanics, that nothing is entirely "frozen", but must have a minimum energy. However, quantitatively, this idea does not match the predictions of quantum mechanics (more precisely: quantum field theory) by many orders of magnitude. Alternative ideas include all sort of "exotic" physics, which involve new elementary particles that are not yet discovered.
Thus, the fact is that in terms of mass/energy, about 70% of the universe is made of "dark energy", which no one really understands. Another ~25% is made of "dark matter", which nobody understands either. Only less than 5% is made of "normal" matter, which we think we understand (or, at least, measure...)