[EForEndeavour]

Would this be the Dark Forest Hypothesis [1], with the added twist that the hidden civilizations are so advanced that they've "cloaked" themselves such that they only show up as what we currently interpret as dark matter?

Aside: dark matter and dark energy are pretty distinct concepts by my reading [2]. Dark matter interacts only gravitationally and is currently the leading explanation for things like galaxy rotation curves (visible mass is far too low to support observed disk rotation speeds, so non-luminous, i.e., "dark" matter must exist to make up the difference) and gravitational lensing by galaxies (again, visible mass is too low to produce observed lensing). Dark energy is, well, something that accelerates the expansion of the universe.

[1] https://en.wikipedia.org/wiki/Dark_forest_hypothesis

[2] https://science.nasa.gov/astrophysics/focus-areas/what-is-da...

[simne]

> Dark matter interacts only gravitationally and is currently the leading explanation for things like galaxy rotation curves

Not exact. For so weak technologically civilizations such our Terrestrial, now could not detect many distant objects which does not emit EM spectrum.

For example, only small fraction of known planets outside our Solar system, seen on telescopes directly, most others detected by indirect means - measuring difference of speed of star (by Doppler shift), or measuring changes of brightness of star, when big planet eclipse star (most cases partially).

In many cases, planets orbits are not so fortunate to us to detect them, and these planets for us are dark matter also.

To be exact, for us "normal" matter are only classic stars and some other objects like Black holes when they "eat" something.

Astronomers already made calculations, based on assumption, that Solar system is more or less typical (we know approx weights of matter of Sun, Planets, Asteroids, Star dust, etc), and seen, that all we know could been about half of dark matter, but other half we could not explain, and this is huge number.

[hollerith]

>planets orbits are not so fortunate to us to detect them, and these planets for us are dark matter also.

"Dark matter" has a technical meaning in astronomy, and this is not it.

[simne]

You confused PHYSICS definition with ASTRONOMY definition, they are different.

For ASTRONOMY, dark matter is just what they could not see, but have gravitation features. And as I said, for astronomy, ~50% of dark matter is nothing special, but for example, only extreme models could suggest tens of thousands Black holes (to fill 50% of emptiness), because of this, they conclude, that exists something other matter which we don't know.

For PHYSICS, technical meaning of dark matter is special type of matter.

[hollerith]

But the person you replied to was using the physics definition, and you reply as if he used the astronomical definition.

[simne]

We have confusing and extremely sophisticated context. It involving wide spectrum of less complex contexts. Physics and astronomy, depend on from what direction we look on it.

Good that we found understanding on this.

[choilive]

Shouldn't we be able to detect the civilizations that have been broadcasting EM radiation before they got quiet? Whether they went silent voluntarily or involuntarily - unless every single technologically advanced civilization had the foresight to develop cloaking before becoming spacefaring (which seems unlikely) we should be able to detect their signals within some window of time.

Humans also have been broadcasting EM out into space for a bit now.. so either these hostile aliens are on their way right now, they are too far away to have detected us yet, or they don't exist.

[simne]

Humanity just very short time have instruments to hear sky.

Visual observations exists much longer, but they are limited to just our galaxy, Milky way (more distant sources are not bright enough to see them with naked eye).

And even most obvious mask ideas (and development), which we see on our current sci-tech stages, very fast move spectrum to infrared, which is near impossible to observe via thin atmosphere. And even now, I hear about developments of laser cooling, which could emit extreme amounts of heat into some direction, where nobody will see it.

For example, now Earth communications conversed to Ultra high frequencies and very high gain antennas (3G, 4G, 5G, emit only very narrow beam to receiver and are very short range), and to fiber-optic communications, which emit just infrared from heat. Also, Ultra high frequencies very quickly absorbed by moisture, so also effectively converted to heat infrared.

So, right question should be, how much time industrial civilization emit observable to others emissions.

And imagine, what could alien civilization learn about Earth from locations, where clouds are rare, as others they could not hear :)