Myriatite is a mineral existing very rarely in some paraverses that has the unusual ability to self-replicate in almost any environment. It is also somehow composed of matter, dark matter, and dark energy, and seems to passively warp reality around itself. While its actual properties are difficult to describe, its function is simple: advanced civilizations use myriatite to "revive" universes ended by heat death or vacuum decay.
When placed into the universe in question (inserted carefully from the outside to prevent equipment damage, if the universe became a true vacuum), it will immediately begin spreading out at approximately the speed of light, creating a growing multicolored sphere. It appears to cancel out vacuum decay as it grows, and will not stop unless it is completely destroyed. Once a ball of myriatite grows fairly large, various destructive devices like von Neumann probes or strangelets can be used to eat away the inside of the ball at lightspeed or slower, which leaves the myriatite to create an expanding shell inside which it is much safer. Given enough time, entire stars, galaxies, and megastructures can be placed inside the new habitable space, and life can continue as normal.