So we have two mysteries: First, where is all the matter we can’t see except by its gravitational influence? The paucity of observed microlensing now virtually rules out that it’s black holes or similarly compact astronomical bodies; while searches for weakly interacting massive particles have come up dry. Second, given the billions of years since stars and planets first formed, even rare interstellar civilizations should have colonized most of our galaxy and presumably the same in other galaxies. Could dark matter be alien habitats and machines in deep space, or is there anything that immediately shoots down this hypothesis? I’ve heard it claimed that there are reasons to believe that dark matter cannot be baryonic, or anything that used to be, but I don’t recall the reasoning.
First off-the-cuff thought is that the universe would be ablaze with their waste heat, invisible to the eye but not to the right telescopes.
There is five times as much dark matter as there is baryonic matter.
But I suppose you could dispute whether this immediately shoots down the hypothesis or two seconds thought is needed.
Electrons and neutrinos cannot be baryonic but they exist and we know that. I am not sure why dark matter would have to be baryonic or how that relates to aliens. Can you explain any other effects on the universe scale or even solar system scale that is caused by life by active agency?
Note that there are other threads on the Fermi Paradox inferred by your post, but it makes a number of assumptions and is a fun thought experiment but doesn’t fundamentally prove anything outside of that.
There is simply no priors to even suggest that some intelligent agent would require some form of evidence that should be considered a possibility. Without a comprehensive theory that can make predictions or explain observations it wouldn’t even be considered.
You would also have a problem explaining events close to the big bang where nucleosynthesis wouldn’t have happened to even produce the elements in quantities to support this type of life.
Dark matter is not really a thing. The only way we know it exists is because galaxies spin at rates that are not accounted for based on how much matter we can see. So something is outta wack.
One the other hand, a Dutch guy named Erik Verlinde put forth an idea called “emergent gravity”, in which gravity itself emerges from quantum relationships between tiny things. It might be thought of, perhaps, as a side-effect of entanglement. Somehow he does the math in a way that at great scale, the force effect of gravity goes from inverse-square to just inverse (drops off considerably less quickly).
Verlinde’s model is difficult to test – but then, so is dark matter. It could be that dark matter is just a blind alley. Which would explain why it is so dark.
We can tell, based on the relative abundances of light isotopes (H-1, deuterium, He-3, the various Li, etc.) just how abundant protons and neutrons were in the early Universe (this is called “Big Bang nucleosynthesis”), and based on that, what the density of baryonic matter (that is, matter that gets most of its mass from protons and neutrons) should be now. That present-day density of baryonic matter isn’t too much higher than what we see (i.e., there is some baryonic matter that we don’t see, but not all that much), but it’s much less than the total density of galaxies or of the Universe. Thus, most of the dark matter (indeed, most matter, period) must be non-baryonic. And while there are some forms of non-baryonic matter known (neutrinos, for instance), none of the known types come close to the total, either. Nor is there any expectation that non-baryonic matter could self-interact strongly enough to form non-gravitational structures.
Most likely, dark matter is made up of one or more types of particle that has yet to be discovered. And yes, we’ve looked for such particles and failed to find them, but we’re really not at a point where that’s surprising: There are many, many possibilities for such particles which have been predicted by various models, but which we wouldn’t have been able to detect with any experiment so far, and that’s leaving off the possibility that it’s something that hasn’t been predicted (as Gell-Mann said when the muon was discovered, “Who ordered that?”).
eschereal, there are several different completely independent lines of evidence that point to the existence of dark matter. Every so often someone comes up with some new model of gravity that purports to explain one of them, but nobody’s ever been able to come up with anything that explains all of them, and most of these alternate models don’t even manage to account for everything that gravity does with known sorts of matter (you get things like the Solar System being unstable on million-year timescales, or binary stars being impossible in the outer reaches of galaxies, or the like).
It may not be their devices, but the effects of their ability to warp space to allow interstellar travel - which to our understanding may involve the ability to create ‘synthetic’ gravity. It would be interesting if that were true, as the universe’s expansion might be halted/reversed by it evolving life everywhere to one day create artificial gravity to pull it all together again.
This is a very good reason. There is five times as much dark matter as visible matter; if there were five Dyson Spheres to every visible star, then we would see the Dyson spheres radiating infra red everywhere. On the other hand, perhaps these aliens prefer to be cold and use energy sparingly. That means that their constructs must be very either extremely large and cool Dyson Spheres radiating at the same temperature as the microwave background, or they are small, cold, numerous objects with the same total mass. Either way these objects would be visible as dark, transiting objects passing in front of distant stars.
They would have had to relatively evenly distributed that matter between every observed galaxy.
If the universe is a simulation, all matter is an alien construct. And Quantum Mechanics is an elaborate practical joke.
I just came across today the concept of “mirror matter”, which among other things is a possible candidate for dark matter.
Basically mirror matter just differs in “handedness” WRT P-symmetry.
Unlike antimatter, mirror matter would interact weakly with regular matter; pretty much just gravitationally. But it could have more complex interactions with itself, possibly forming molecules or whatever.
I know it belongs to the long list of hypotheses with zero support right now, but if the OP wants to have fun imagining the possibilities, well seems mirror matter is one way.
Or the aliens are composed entirely of dark matter, interacting with all the dark energy in the universe. And we can never engage with them because they occupy the other 87% of the universe that we cannot perceive. We might not even be able to discover their existence because the bright side of the universe that they occupy has not direct connection to our dim segment (there is far more “dark energy” than anything else in the universe).
Of course, since there must be many more of them, that makes us the “aliens”.
And they tried to destroy the Milky Way galaxy by turning the central black hole into a naked singularity, but a hyperadvanced AI instead turned it into a ludicrously-powerful generator to use to make war against them, so now they’re using hyperspace death rays to try to destroy that generator.
What will you find in a toilet, after an alien has used it?
Get it! DARK MATTER. Eheh … heh … eh …
The trick is to use it as a gateway to a new universe unpopulated by photino birds.
Now, onto the ‘dark energy’ problem…
Wait, Stranger, you didn’t get that reference? Start reading; you’ll enjoy it.
Note that DM would have to pretty much be aliens constructs. All the galaxies we’ve measured have it. Some so far away from each other that there is no known way for a single alien civilization to have traveled to each and done their constructions.
And for civilizations to have arisen in all these galaxies to such an advanced level and developed the same technology, etc. boggles the mind to an incredible extent.
Yet, somehow, the galaxy-modifying civilization for the Milky Way hasn’t affected us at all over the last 4+ billion years.
The failure to identify dark matter is becoming as embarrassing as the luminiferous aether.
No it is not embarrassing.
It is a known unknown. We see the effect, we have no idea what is causing it, although we can rule out some possibilities.
Aether was not an embarrassing idea. It was a logical idea that was supported by lots of evidence, the evidence being: waves travel through a medium, light is a wave, therefore, light travels through a medium, but what that medium is we have no idea other than to give it a name.
It might be that there is no such thing as “dark matter”, and that instead our theories of gravity are woefully incomplete. But if they’re so incomplete, why do they work so well to describe stuff like the solar system?
Dark matter is a logical idea that is compatible with the known laws of physics. We know there are particles that are weakly interacting, like neutrinos, so it is easily possible that there are other types of weakly interacting massive particles. We just haven’t found any, but that’s because they’re weakly interacting. So if we want to find them, we have to figure out ways to make them interact which isn’t easy.
Again, “dark matter” is just a placeholder, same as “Aether” was a placeholder. That doesn’t make it embarrassing. If you think it’s so embarrassing, then why don’t you become a physics professor, figure everything out for us, and set us all straight?
This is how science works. We start out with unknown unknowns, then we find out what we don’t know and it becomes a known unknown, then we figure it out and it becomes a known known, except when we got it wrong and then it’s an unknown known. In any case, the existence of known unknowns is not embarrassing, it’s exciting, because physics PhD candidates gotta eat. And besides if we knew everything we’d have to shut down the patent office, like that one guy one time that didn’t actually say that.