When you die, you get buried, cremated, freeze dried, cannibalised or even left for vultures to be scavenged, depending on your beliefs and cultural circumstances; but what has that got to do with the death of the Universe?
As a living organism, we are a well organised set of atoms and molecules with well structured biochemical processes, making our entropy very low. As that organised set is broken down into indistinguishable fragments of atoms and molecules by any funeral method, the entropy increases substantially [read more about Entropy, on Why Do Things Tend To Go Wrong?]. In cremation, entropy rise is quick, and in burial, the body is slowly digested and decayed, increasing your entropy at a slower pace. As neither matter nor energy (both of which make you up) can be created or destroyed, complex biological matter (chemical matter essentially) eventually get converted into smaller molecules while releasing energy as heat.
The same happens while consuming food. Eaten meals are broken down by various biochemical processes inside the body and get converted into stored energy, Carbon Dioxide, water, and some bio-mass, releasing chemical energy and heat, thereby increasing the entropy of the food. Another analogy to this are machines. They consume energy, and eventually release them as some useful work (motion), sound, vibration, light, and heat.
Useful motion work/ motion/ vibration will eventually convert into heat because perpetual motion machines cannot exist. But where and how does the light disappear? And how do sound and vibrations vanish in thin air? Light heats up the object it hits onto, giving off its energy, and sound gets dampened in a medium by giving off its energy to atoms in that medium, releasing heat. Any biological, chemical or physical process eventually produces heat as a by-product.
But why heat and not any other forms of energy? Is heat energy our Universes’ favourite or a mere coincidence? Thermal energy, is an indication of the internal energy of a system, that is, it is the most primitive form of energy. This has an implication that the conversion efficiency of other forms of energy to heat energy should be the highest (~100%), and that of the reverse, or even between other energy forms is lesser. Thermal energy or heat as a by or final product is constitutional to Thermodynamics. Some examples of energy conversion efficiencies are shown here.
Simply written, Physics is taking the least resistive path to increase its Entropy to maximum, which is through heat. So How Will The Universe Die (does it have to)?
Efficiencies less than 100% denote parasitic loss, in terms of Chemical, Radiant, Mechanical or Sound energy, which eventually also convert to Thermal Energy. This is the most stable configuration for equilibrium, drawing parallels to the Minimum Total Potential Energy Principle, which asserts that a body shall deform or displace to a position that minimises the total potential energy (Suvranu De); and the lost potential energy being dissipated as heat, for example a stretched spring has a tendency to return to unsprung state, while it gets heated up due to internal friction.
In 100 trillion years (universe’s age is just 13.8 billion years), the continuous and accelerated expansion of the universe will lead to exhaustion of gas particles in proximity to each other to clump together for new star or planet formation (Stelliferous Era). Further, all existing stars will burn out (Degenerate Era) and black holes will govern the universe for trillions and trillions of years (Black Hole Era), but they will also eventually evaluate due to Hawking Radiation, and all matter will reach absolute zero temperature (Dark Era).
The universe will tend to its maximum disorder, all matter will get evenly distributed and no energy transfer process can occur because of the thermal equilibrium.There will be no light, no energy transfer due to thermal equilibrium within the universe, and we will be in a state of coldness and darkness, called the Big Freeze (heavily simplified explanation).
Of course this is one prediction of the many proposed theories (Big Crunch, Big Rip, Big Bounce, False Vacuum and more as our understanding of dark energy is very limited and dependence on the shape of the universe), and only
time (Science) can tell.