[vee-kay]

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.

[thyristan]

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.

[vee-kay]

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