The same way we find the disease: by the effects the aliens have on their surroundings.

You are missing the point, in our arrogance to believe we understand things, we do not really understand them. The drive to understand more is powerful, and has lead to huge advances, but we still have a long way to go.

I agree that we still have a very long way to go. I disagree that the areas of the universe that are still mysterious to us are likely to be hiding places for aliens.

> We still don't understand what causes many diseases, or how to heal many wounds.

Right, but we still know the diseases and wounds exist.

Sometimes, you have to wait until you're dead or dying to know they exist. I'm not convinced that humans have mastered perception after mastering writing 5 thousand years ago.

You're partially right, but missing something important.

Where you're correct: Aliens that are only found on planets would be almost impossible to spot, and even if we look at them and see the consequences of their existence on their atmosphere we may interpret that as non biological in origin.

What you're missing: However, if they are expansionist and can build space habitats and automation of the sort we have reason to believe is possible, the question is "why are there visible stars? Why are they not all already surrounded in Dyson swarms?" which is hard to miss regardless of underlying chemistry or biology.

So far as we can currently understand, it seems that about 96% of the matter-energy of the universe is stuff that we cannot see, detect, or explain. Dark matter/energy.

There's more than enough room for fanciful explanations.

Except that dark matter doesn't even interact with itself. Particles of dark matter just pass right through each other rather than clumping together to form complex molecules, rocks, planets, etc.

A Dyson swarm emits approximately same energy as its host star, just in infrared. Dark Matter could, I guess, be Matryoshka brains organised carefully to not hit each other by their keepers, so long as they are also actively "gardening" what we can see so they never ever get Dyson-d.