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Strange matter is an ultra-dense phase of matter that is theorized to form inside particularly massive neutron stars. It is theorized that when the neutronium which makes up a neutron star is put under sufficient pressure due to the star's gravity, the individual neutrons break down and their constituent quarks form strange matter. The star then becomes known as a "strange star" or "quark star". Strange matter is composed of strange quarks bound to each other directly, in a similar manner to how neutronium is composed of neutrons; a strange star is essentially a single gigantic nucleon. A strange star lies between neutron stars and black holes in terms of both mass and density, and if sufficient additional matter is added to a strange star it will collapse into a black hole as well.

Some theories suggest that strange matter, unlike neutronium, may be stable outside of the intense pressure that produced it; if this is so, then small substellar pieces of strange stars (sometimes called "strangelets") may exist in space in a wide range of sizes all the way down to atomic scales. There is some concern that ordinary matter, upon contacting a strangelet, would be compressed into additional strange matter by its gravity; strangelets would therefore be able to "eat" any ordinary matter it came into contact with, such as planets or stars. They would then increase in size, because after eating all of the protons in the nucleus they would gain a positive charge, which would then attract the electrons, surrounding it with a cloud of negative electrical charge. When the charge became great enough, the strangelet would begin "creating" electrons and positrons around it. Any passing nucleus would have its electrons annihilated by the positrons and the negative charge of the strangelet would pull it in. Mainstream physicists, while acknowledging this possibility, tend to downplay its probability.

Strangelets are thought to have a net positive charge, which is neutralized by the presence of degenerate electrons extending slightly beyond the edge of the strangelet, a kind of electron "atmosphere." If a normal matter atomic nucleus encounters a strangelet, it will approach until it begins penetrating this negatively charged atmosphere. At that point it will start to see the positive electrical potential and be repelled from the strangelet. Sufficiently energetic nuclei, or neutrons (which are unaffected by electrical charges), can reach the strangelet and be absorbed; the up/down/strange quark ratio would then readjust by beta decay.

Strange matter is one candidate for the hypothetical dark matter that is a feature of several cosmological theories.

Strange matter is largely theoretical at this point, but observations by the Chandra X-ray Observatory in 2002 detected two candidate strange stars, designated RX J185635-3754 and 3C58, which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than they should, suggesting that they are composed of material denser than neutronium. However, these observations have been under attack by researchers who say the results were not conclusive; it remains to be seen how the question of strange star existence will play out.

There has also been some evidence that quark matter may have been produced in particle accelerators at CERN in 2000.

In May 2002, a group of researchers at the Southern Methodist University reported the possibility that strange matter may have been responsible for two unexplained seismic events recorded on October 22 and November 24 in 1993; they proposed that two strangelets of unknown mass moving at roughly 400 km/s had passed through Earth, generating seismic shock waves along its path. The members of the group were Vidgor Teplitz, Eugene Herrin, David Anderson and Ileana Tibuleac. It has been suggested that the International Monitoring System being set up to verify the Comprehensive Nuclear Test Ban Treaty (CTBT) may be useful as a sort of "strangelet observatory" using the entire Earth as its detector; the IMS will be designed to detect anomalous seismic disturbances down to 1 kiloton of TNT's equivalent energy release or less, and could be able to track strangelets passing through Earth in real time if properly exploited.

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