The search continues for dark matter, the mysterious stuff that makes up the vast majority of all matter in the cosmos. Although nowadays they're most likely undetectable subatomic particles, in the universe's beginnings they could have formed super-explosive dark stars.
Dark matter is, in brief, a huge group of unidentified matter that has resisted all efforts at direct detection, but must exist because we can measure its gravitational pull on regular matter.Dark matter accounts for probably about 85% of all matter in the universe, and some estimates put it as high as 98%.
One of the unusual properties about most candidates for the dark matter particle is that they are their own antiparticle, meaning any two dark matter particles interacting in close proximity would annihilate each other in a matter/antimatter reaction. The universe is now big enough that this almost never would occur naturally, but when the universe was younger and smaller, it was possible for dark matter particles to accumulate together and form the nucleus of dark stars.
Dark stars are something of a misnomer - unlike dark matter particles, they would be visible and actually quite bright because they would still mostly be made of regular matter. In fact, the dark stars would have been the brightest bodies in the young universe, as the constant annihilation of dark matter particles would release massive amounts of energy. The star would burn through its dark matter nucleus very quickly, and then a new nucleus of normal matter would form, starting up the more familiar nuclear fusion process inside the star. It's possible these former dark matter stars are still around in the modern universe.
Whether dark stars ever existed depends on which hypothetical particle makes up dark matter. According to research led by Paolo Gondolo at the University of Utah, the axion is a main contender for the dark matter particle, but it wouldn't be able to form dark stars. As such, if we ever observe dark stars, that would probably eliminate the axion as the main dark matter particle. On the other hand, some of the Weakly Interacting Massive Particles, or WIMPs, would be perfect to form dark stars, particularly the supersymmetric photino.