Imagine the coldest place you possibly can. Inside it, the particles that make up matter move as slowly as you can possibly imagine, approaching the quantum limit of what it means to be truly at rest. There will be no major interior heat sources nearby for those particles inside to absorb; there will be no significant exterior sources of energy heating them up from the outside.
Physically, that means you need to be as far away as possible as you can from all sources of moving particles and radiation. You’d want to be at as great a distance as you could from stars, galaxies, and contracting gas clouds. You’d want to screen out any external sources of photons. If you headed to the deepest recesses of intergalactic space, shielded from starlight, the only thing that would heat you up would be the Big Bang’s leftover glow: the cosmic microwave background at 2.725 K. And yet, our own galaxy has a place — the Boomerang Nebula — that’s even colder than that.