The universe is expanding at an increasingly faster rate, and much of it we cannot "see." Current explanations for these two phenomena gave rise to terms such as dark energy and dark matter. The former is very mysterious and little is known about its nature, and the latter is understood a little more.
In 1999 I attended a talk on the observed acceleration in the rate of expansion of the universe at Harvard University. Dark energy is believed to be responsible for this accelerating expansion, and seems to permeate all of space. Just by virtue of the void of this "space," it may be filled with a constant energy that acts repulsively, sort of a powerful negative pressure. This is typically referred to as the cosmological constant. Quintessence is a scalar field quantity in quantum mechanics that serves as another proposed form of dark energy. The difference is that the energy density varies in both time and space. Of all the mass-energy throughout the entire universe, dark energy accounts for nearly 3/4 of the total.
In the mid 70's, a world-renowned woman astrophysicist, Dr. Vera Rubin, found that the visible matter (stars, gas and dust) spreading further out from the core of galaxies was moving as fast or faster than the matter near the core. This was opposite to what was expected. Studying these "rotation curves" in many galaxies revealed that this was happening everywhere. The cause is believed to be a huge amount of mass acting gravitationally that isn't detectable. This "missing" mass obviously doesn't interact with electromagnetic energy and seems completely transparent. That is why we call it dark matter.
Some of this dark matter is accounted for by ordinary types of matter containing atoms. This baryonic form includes candidates like brown dwarfs, gas that is not luminous or glowing, and Massive Astrophysical Compact Halo Objects (MACHOs) such as black holes and neutron stars. Neutron stars are the stellar corpse remnants of supernovae that are extremely dense, often having the mass of a normal star yet only a fraction of its size. If they emit electromagnetic radiation, such as X-rays, and spin giving pulses of light (commonly referred to as the lighthouse effect), then they are known as pulsars. A well-known example of this is the Crab Nebula Pulsar in the constellation Taurus the Bull. The supernova (within our galaxy) that made this was seen in the year 1054 AD. It was so bright that it was plainly visible during the day and outshined the full moon at night. The vast majority of dark matter, however, is considered nonbaryonic. This form includes Weakly Interactive Massive Particles (WIMPS), such as neutrinos and other hypothetical elementary particles. Since their presence can only be inferred from gravitational effects on visible matter, these elusive dark components of the universe are very little understood.