There is a concept known as strange matter which has been theorized based on our mathematical models of nuclear physics, though never observed. It would be a sort of "liquid" made of quarks which aren't confined to protons and neutrons, and it's been theorized that forms of it could be stable at low temperatures and pressures--for example, this paper says "Witten has pointed out that strange quark matter might be stable at a zero temperature and at a zero external pressure." $\text{*}$ (In fact, the analysis indicates it should be more stable than ordinary matter under these conditions since as mentioned here it would be the 'ground state' of matter made of quarks, meaning it would have lower potential energy than quarks confined to protons and neutrons. As discussed in this answer from the physics stack exchange, the idea is that ordinary matter is "metastable", having a potential barrier that tends to prevent it from decaying to strange matter unless it temporarily gains enough energy to hop the barrier, or unless you wait a sufficiently huge time for it to get through the barrier via quantum tunneling)
It's mentioned on p. 29 of this paper that negatively charged bulk strange matter would have disastrous consequences in the real world, since "ordinary atoms would be attracted to it and absorbed" (converted into strange matter themselves), but that for positively charged bulk strange matter, "a Coulomb barrier prevents this system from absorbing the nuclei or ordinary atoms" (and there are theoretical and observational arguments against negatively-charged strange matter being stable enough to 'infect' ordinary matter in this way). It's also mentioned that "since it is very dense even small chunks cannot be supported by material forces at the Earth's surface." This would suggest a problem with using such matter for construction on planets, and would also greatly add to a spacecraft's mass which would increase the fuel needed to accelerate it, but perhaps one could imagine using it to plate space stations as a form of armor (and I'm also not sure how think a later of strange matter could potentially be, perhaps thin enough that the mass wouldn't be so large even for a largish area being plated). Another interesting science-fictional use for very small bits of strange matter could be to create some form of ultra-tiny machines much smaller than nanotechnology, the notion of "femtotechnology" discussed in this article, which could function at low pressure and temperature.
