Space

Not to mention the problem of what life is even supposed to do beyond a certain point of development. The depressing fact is that there is a finite amount of knowledge to be gained, a finite amount of resources to harvest, a finite diversity of life to contend or thrive alongside with. Once a pocket of life in this massive universe begins to run out of things to do and stagnates, then what? What is there to think about; to feel; to experience?

There's little point in exploring space if one know how this universe works. One knows the rules, knows all the ways it can play out, and there's no surprise waiting on the other end of any venture one can imagine embarking on.

That's my theory. The Great Filter is just depressive boredom. We don't see other life because by the time a civilisation is able and ready to spend thousands of years travelling through deep space, they'll have already lost any motivation they might have had to do so.

I suspect that there's at best a very short window wherein a species is both knowledgeable enough to dream of space exploration and technologically capable of sending any significant amount of artificial constructions out there.

Not to mention that anything an alien species might send into interstellar space is unimaginably unlikely to be recorded exactly at precisely the moment they pass another lump of matter - especially if the window is as short as I fear.