Metamesonium is an extremely volatile and dangerous material found in unstable multiverses in the instants before they rip themselves apart. It is composed of eight electrons orbiting a core of eight mesons bound together at slightly different temporal "locations"; at the impossibly high energies needed to form metamesonium, time itself can bend in unusual ways and allow particles to stack themselves in slightly altered timelines. Some extremely advanced civilizations are able to harvest metamesonium by arresting all spacetime motion just before a multiverse is destroyed and collecting the material from the dying universes inside. The material has two main properties that make it worthwhile.
The first is that it is an infinitely-replicating fuel supply for subatomic reactions; with proper timeline manipulation one can generate arbitrary quantities of metamesonium and carefully break it apart into safer particles, releasing energy to perform various tasks. Metamesonium reactors can be very small, even the size of individual molecules, which means that nanites might be able to use them if they need absolutely unimaginable amounts of power.
The second is that if one of its timelines is altered in some way it will rupture and split into an infinitely-scaling explosion that can consume multiverses, megaverses, or even low-level archverses. Thus, in the wrong hands it is a weapon of mass destruction beyond human comprehension.