Final answer:
A time far in the future of the universe, when the density of photons and elementary particles will be very low and all the stars have burnt out, is known as the heat death of the universe, where energy is evenly distributed and no workable energy remains.
Step-by-step explanation:
The 'heat-death' of the universe is when the universe has reached a state of maximum entropy. This happens when all available energy (such as from a hot source) has moved to places of less energy (such as a colder source). Once this has happened, no more work can be extracted from the universe.
This scenario is based on the Second Law of Thermodynamics, which indicates that the universe will reach a state of maximum entropy, where all energy is uniformly distributed, and there are no gradients to sustain motions or life, leading to a 'dead', quiet universe with no workable energy sources.
As stars exhaust their nuclear fuel and galaxies go dark, the universe will become unable to host life as it disperses into a cold, dilute state over trillions of years.