In addition to matter, Einstein's theory permits another source of curvature called the cosmological constant . Unlike ordinary matter, which gets less dense when it is spread out, the cosmological constant represents a kind of matter with the curious property that it remains at constant density when it is stretched. Recent observations of light from bright stellar explosions called supernovae seem to indicate that space-time actually is curved in such a way that the curvature appears to be caused in part by a cosmological constant. In particle accelerators today it is just as easy to produce antimatter as it is to produce matter. Very small quantities of antimatter are also produced inside your body every second, but not enough to harm you.
Antimatter is essentially the same as matter, but with some properties opposite.
In antimatter , however, the atom has a positive electron and a negative nucleus. The universe today contains a noticable excess of matter versus antimatter. To see this, try to shake hands with the person next to you. If he or she is made of antimatter, you will both annihilate in a big flash. The reason for the observed asymmetry between matter and antimatter has so far not been resolved, but it is believed that the origin is cosmological, i.e. that it results from some particle-physics process in the very early universe.