True, but there is thought to be a finite amount of matter + energy, which cannot be created or destroyed. And since it is spreading out from an original dense point, it stands to reason that there would be a vacuum area that it has not reached yet.

It may as well as it is unreachable. It hasn't existed forever hence only a limited amount of space is closer to us than the age of the universe.

John Green: Yikes!

Dark Forest Theory is probably wrong. In-universe, the series unknowingly undermines it with communication tech that can transmit instantaneously. That would take away the assumption that civilizations can't effectively communicate over interstellar distances and build trust.

In reality, it's something of an extension of the "every individual for themselves" mindset of evolution--something White Supremacists have loved. Kin Selection Theory does away with that. There is a basis for building trust and working together within evolution. The precursor ideas were even done in Peter Kropotkin's "Mutual Aid: A Factor of Evolution" over a century ago. Kin Selection Theory put a mathematical foundation on it.

I like the book series as literature, and the Netflix series has been OK so far (not great, but OK). Liu Cixin himself, however, has some really shitty opinions that come through the text.

Or maybe they were just bored, and wanted to make a cool new celestial object called a black hole.

I wouldn't worry too much about it. Anything capable of altering fundamental physical parameters like that will be unknowable to us. We'd be like bacteria are to a human.

Take a shortcut through hell, eh?