You guys know about entropy and the heat death of the universe? Depressing stuff.
Anyways, if you don't, you know how, according to the second law of thermodynamics, heat energy will always spread to where there is less of it. For example, if you put a blast of hot air in a box of cold air, the temperature in that box will eventually reach an equilibrium. Heat energy transmits in three ways: conduction, wherein it passes through solid objects; convection, wherein the heat source heats some air whcih rises out of the way, exposing more cold air to heat, which rises out of the way, getting displaced by more cold air (to give an example); and radiation, whereupon the heat energy results in kinetic energy of particles (including light), which spread along, potentially to heat up another object. Radiation requires no atoms, only any particle, like an electron or a photon.
The theory is that, through means of conduction, convection, and radiation, heat will keep spreading to cold, and, like the air in a box analogy I gave earlier, stars, because they die at a faster rate than they are produced, eventually die out, and black holes evaporate via hawking radiation. Atoms dissolve into basal particles. And the universe will eventually reach a state of total thermal equilibrium, whereupon all the heat and particles in the universe are so perfectly dispersed that nothing can interact with each other any more. After all, absolute zero is the absence of all atomic activity.
How long could life survive on the death journey to total thermal equilibrium? This includes both life in general, and humans (off the topic of spec evo, but still).
This ties in with space-faring organisms. Maybe they have super slow metabolisms, so minimal food can sustain them.