What do we do if SETI is successful?

The reason for the paranoia is that the risks are maximal. Any planet can destroy any other planet by accelerating a small projectile at it, so long as it achieves sufficient kinetic force. The projectile can be so tiny as to be effectively undetectable until it’s too late. So you have a situation where everyone you meet is carrying WMDs, and you can’t guarantee you’ll be able to get revenge if they fire first. Finally, every actor knows the predicament and nothing else about the other actors. If you don’t know who is on the other side of the radio transmission, but you do know they can destroy you immediately and without consequence, and you know they know you can do the same to them, the only rational choice is to shoot first, because you’d better not shoot second!

But the reverse it true too. You can't be sure you got every one of the other species. Especially with the transmission times at light speed. In the books, they use these dimension collapsing bombs to eradicate everything, but, um, those don't actually exist. Even in the books, humanity still survives in little groups, some bent on vengeance.

Much safer to make friends or coinvestors, slaves at the very least. Get them all to buy in and police themselves. Better yet, you take that one rare thing, life, intelligence, and put it to work for you. Make the aliens you've just contacted be a part of the pyramid scheme

Either life really is extremely rare (most likely), or intelligent life is, or it isn't actually trivial/correct to destroy alien planets. If the galaxy were actually like that we would have been toast a long time ago. In reality dark forest is a generate thesis since it implies we're alone, so no aliens anyway.

> slaves at the very least

If you think that's a relevant upside, then some of them think so too. Well we better just start shooting up all the exoplanets as soon as we can. Not take any chances.

We really don't know much about the universe and it is too vast and unfathomed. Scientists computed the mass of all matter and all energy of this Universe, but their calculations told that all this stuff comprises merely 5% of the Universe, the remaining 95% of the Universe is said to made of anti-matter and anti-energy, about which not much is understood.

So there's a good chance that aliens may be made of anti-matter and using anti-energy. But even if they tried to communicated with rest of universe with such anti-energy-based technology, we humans simply may not be detecting it or interpreting it yet, and we may still be waiting for that elusive signal (energy-based) indicating advanced intelligent life.

I didn't consider dimension collapsing bombs realistic either. It is the objects sped up to a fraction of light speed which were more scary. The cylons carpet bombed the planet with nuclear bombs which was plausible.

Nuclear carpet bombing is only sensible for short-range attacks. On longer ranges, fast projectiles are the way to go, because you need to accelerate your weapon anyways. And by just putting all the energy and resources into acceleration, you do recycle the motion energy as destructive energy.

Nope, not "anti-". The 5% are visible "bright" mass and energy. "Bright" meaning that we can see it through telescopes, by various wavelengths of light, particle emissions, gravity waves. The rest is "dark matter" and "dark energy", which just means that we see signs of it being there, because the bright matter around it behaves differently. But we don't directly see it in a telescope of any kind. Those "dark" things are stand-ins for our not understanding: Those could be real matter and energy that we just cannot see for some reason. Or those could be problems in our cosmological theories, like gravity working differently on large scales, the expansion of the universe being different, or physical constants changing over time. We just notice that things are off and that we should see more matter and more energy than we do.

Most theories that involve "dark matter" being ordinary matter like tons of neutron stars, huge clouds of dust, bazillions of asteroids or dark planets have been checked for and excluded. So if there were "dark matter" aliens, they really would be completely strange in that they aren't even made from the same kind of matter, but from maybe particles that we don't even know about. But if those hypothetical dark matter particles were capable of this kind of organisation, like clumping together into stars or planets, we would have probably seen those by now. So extremely strange, and improbable imho.

Btw. anti-matter is not "dark matter" in this sense, and dark matter being anti-matter was excluded very very early on by a simple observation: anti-matter and matter, when they come into contact, react in an annihilation reaction. E.g. an electron and anti-electron annihilate into two photons of a characteristic and exact 511keV energy. All other particles and their anti-particles also do this and exhibit their own characteristic energy. Any contact between a region of matter and region of anti-matter in space would radiate in these energy signatures, something which is very easy to detect. Dark matter is known to exist within galaxies, even within star systems, so this kind of contact zone would have to be there, and would be extremely visible to us.

Anti-energy doesn't exist in our current understanding of physics. Energy is always positive, and in quantum theories energy cannot even become zero, always slightly above zero.

Anyone you encounter in the street might be packing a weapon and planning to shoot you. The only rational choice is to shoot everyone you see first.

This is an example of how rhetoric can hijack people's ability to reason logically.

Oops, yes, you are right. I meant "dark matter" and "dark energy" (comprises 95% of Uinverse), not "anti matter" and "anti energy".