This is not an attempt to explainDark Soulsthrough science, I’m not saying thermodynamics should be canon, I’m just saying let’s use it for comparison, to help us understand the mechanics that are already in place.
If you wish to know more about the actual science behind the actual scientific theory, thisWikipedia pageis a good starting point, andKurzgesagthas some great videos with easy to follow explanations on the matter.

Heat Death End of the Souls Universe
InDemon’s Souls, there was a gray fog spreading out from the kingdom ofBoletaria, threatening to cover the entire world. Inside that fog, there lived demons who absorbed the souls of all living beings. And these hollowed, soulless beings would then decay with the world around them. If the spread of the fog wasn’t stopped, that would be the end of the world as we know it…
Though it will never be confirmed, the soulsborne franchises are heavily implied to be connected, and it has been stipulated that theworld of ancient dragonsinDark Soulsis a direct result of the spread of theOld One’s gray fog, from Demon’s Souls. What I wish to put forth is that, deliberately or not, this is an artistic representation of the “Heat death end of the universe” theory. The end is gone and it will be gone again.

Continuity
Simply put, every system leans toward the state of highest entropy, which can be described as the amount of “unavailable energy” to do work. In our ever-expanding universe that longs for equilibrium, everything decays and dissipates.
“The result would inevitably be a state of universal rest and death. Through an endless space, of action involving the transformation of potential energy into palpable motion and hence into heat, then to a single finite mechanism, running down like a clock, and stopping forever.” -William Thomson, Lord Kelvin of Largs.

But it is possible that, in the far distance, an event called “quantum tunneling” will shatter the equilibrium: a new big bang ushering a new universe, restarting the cycle all over again. And thus, theFirst Flamewas born.
Equilibrium
Besides being a very poetic way to talk about science, this also helps us understand how things work within the souls universe. The gray fog world had balance, but a new system was introduced. The First Flame brought disparity: heat and cold; light and dark. The two systems will attempt to balance each other out, eventually leading to the gray desert where we fightGael. The nextendingof this brave new world.
And at the same time, this new system will be trying to balance itself out, by bringing together all its many parts. A process that can be seen starting inLothricand culminating at the end of the Age of Fire, with all the kingdoms from all the times merging together next to theKiln of the First Flame. This process also applies to the Dark, who is so incessantly trying to fill its own void.

The Dark Chasm of Old would consume everything it could. The shards of theAbysswould consumepowerful kings. Bonfires and itsFirekeepersare, functionally, storage units forhumanity, theDarkwraithsgather humanity through the power of theDark Soul, theLocust Preachersshould be gathering humanity through the power of preaching, and humanity itself gathered aroundManus, voluntarily or otherwise.
“Light and Dark are two sides of the same coin,” unequivocally different, while at the same time remaining part of the same whole. Both parts fighting for supremacy, both sides destined to lose. There can be no victors. Only the inexorable march for equilibrium will settle this war.

If you like this theory, check outthe videowhich contains it along with other theories explaining the connections between the Abyss, the Deep, and the Dark.