In particle physics and astrophysics, the term strange matter is used in two different contexts, one broader and the other more specific and hypothetical:
# In the broader context, our current understanding of the laws of nature predicts that strange matter could be created when nuclear matter (made of protons and neutrons) is compressed beyond a critical density. At this critical pressure and density, the protons and neutrons dissociate into quarks, yielding quark matter and potentially strange matter. 
# A more specific hypothesis is that quark matter is the true ground state of all matter, and thus more stable than ordinary nuclear matter. This idea is known as the "strange matter hypothesis", or the Bodmer–Witten assumption. Under this hypothesis, the nuclei of the atoms we see around us are only metastable, even when the external critical pressure is zero, and given enough time (or the right stimulus) the nuclei would decay into stable droplets of strange matter. Droplets of strange matter are also referred to as strangelets.