[frogeyedpeas]

There's only the fact that we probably just CANNOT perceive extremely advanced civilizations.

An insect crawling around a 75 year old brick house that is covered in ivy and moss will have NO idea that the object it is walking upon is NOT part of it's natural environment. That brick house seems as natural to the environment as the grass, and trees, and rocks, and streams nearby it -- to the bug at least.

Similarly we take our telescope out and see what looks like a natural organic universe with organic galaxies and normal looking stars etc...

Because we don't have solar system sized brains and billion year life spans we are absolutely hopeless to realize that theres' a lot of massive artificial structures in this universe. We're too bug-like to even be able to perceive them from our natural environment.

*we do know of massive cosmic structures like filaments, voids, and the great wall. So it is possible we as humans are starting to notice the "house" in the woods since our theories of physics cannot really explain why these structures exist at these massive scales (we would expect uniformity at those scales). See [here](https://en.wikipedia.org/wiki/List_of_largest_cosmic_structu...)

[jmyeet]

This idea that can't perceive sufficiently advanced civilizations doesn't hold up to scrutiny. It comes down to energy and mass (ie resources). Both of them essentially necessitate a civilization becoming large. You can argue that not every civilization will be expansionist but those that aren't tend to get swallowed up by those that are, at least based on Earth's history.

So for energy, a likely path will be the Dyson Swarm, meaning a cloud of orbitals. Many mistakenly think a Dyson Sphere was a rigid shell around a star. It never was. There's no material, actual or even theorized, that has the rigidity to sustain that. Because of that confusion, many now prefer the nomenclature of "Dyson Swarm" over "Dyson Sphere".

Dyson Swarms have the advantage of creating incredible amounts of living room and solving energy needs with relatively low tech (ie solar). They can also be built incrementally. A cloud of orbitals that capture the Sun's energy with orbitals between Venus and Mars will (IIRC) have a mean distance between them of ~100,000km.

Why is this important? Because the only way to get rid of heat in space is by expelling mass or, more likely, radiating it away into space. You can reuse waste heat to some degree but it's not perfect (because thermodynamics) and you can't totally avoid radiating heat away totally anyway. The wavelength of such radiation is entirely dependent on the temperature. At any likely temperature, that means infrared ("IR") radiation.

So a Dyson Swarm around our Sun would stick out like a sore thumb with a massive IR signature. There's really no hiding it. And we're capable of detecting it.

Conversey, there's really no hiding from any civilization capable of such feats of engineering. Plus any such civilization would be capable of sterilizing the galaxy out of any competition.

Mass follows on from this.

[sandspar]

I don't understand why people fixate on Dyson spheres. Why would a Kardashev type 2 civilization rely on something that a type 0.7 civilization could come up with? Surely in the intervening 1.3 they could come up with something we can't. Dyson spheres are a type 0.7 imagination exercise. A type 2 civilization wouldn't just extend our technology - it would transcend it. Maybe they use dark energy, maybe they gather energy from simulations. We can't understand.

[kulahan]

There are billions of engines sitting around the universe; might as well just use those.

The more advanced versions tend to simply use more radiative heat. they might arrange stellar nurseries, modify stars to be more efficient, or use black holes for engines, but even then you’re still just feeding them stars anyways.

[UltraSane]

extremely advanced civilization would be living around black holes because they are perfect heat dumps (they get colder as you add matter!) and would enable fundamentally better technology.