It doesn't have to be. De Sitter came up with solutions to the equations of GR that were free of ordinary matter but included a cosmological constant that is the effect of an "inherent" curvature of space-time: https://en.wikipedia.org/wiki/De_Sitter_universe
However, the kind of curvature you get from a cosmological constant is not sufficient to explain everything that is observed, so even though modern cosmologies typically have a "dark energy" term (which is another name for a cosmological constant) they require an extra matter term (dark matter) to describe the universe we see.
https://en.wikipedia.org/wiki/Tests_of_general_relativity#De...
I'm more curious about dark energy though. Hopefully in my lifetime, we can figure out what dark energy is. I'm curious what dark energy does to time since it accelerates the expansion of space. The next golden age of science resides in our understanding of the unknown matter and energy that we are currently observing.
1. "Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but accounts for most of the matter in the Universe."
2. "It is hypothesized to be matter that does not react to light."
3. "Astrophysicists hypothesized dark matter because of discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the observable matter (stars, gas, and dust) that they can be seen to contain."
4. "dark matter — that exerts a well-understood gravitational force but neither absorbs nor emits light, and doesn’t collide (as far as we can tell) with protons, neutrons or electrons. So while normal matter (in pink, below) slows down and can even stick together when it runs into other normal matter, dark matter (in blue, below) just passes right through both itself and all other forms of matter."
It sounds to me like a kind of Aether 2.0.
By the same token, we might compare dark matter to neutrinos, which were a theoretical entity invoked to explain a particular set of observations on radioactivity (the shape of the beta spectrum.) Neutrinos turned out to exist.
As such, while the comparison to aether is superficially apt, it is not something we can draw any conclusions from, because the comparison to the neutrino is equally apt.
Science is the discipline of publicly testing ideas by systematic observation, controlled experiment and Bayesian inference. It is not the discipline of testing ideas by making analogies to other ideas. There is a reason for this: making analogies to other ideas has consistently proven to be almost completely useless for creating knowledge of reality, while the discipline of science has been wildly successful.
Nor are the properties of caloric, aether, neutrinos or dark matter "magical". They are merely the ones required of an entity that is able to explain our observations in each instance. In the case of caloric it turned out to have self-contradictory properties, when the full deductive closure of the theory was teased out. In the case of aether it turned out to have properties that made predictions that were false. In the case of neutrinos the required properties made predictions that were true.
In the case of dark matter: we don't know yet, and the only way we will ever know is if we continue on with our program of systematic observation, controlled experiment and Bayesian inference. There is no other way to know.
Dark energy is still a rather large mystery, but it's possible someone will make a big breakthrough at some point.
Because the non-dark matter component of the galaxy formed a disk while the dark matter halo will likely have remained roughly spherical, we expect that Earth is moving through the dark matter halo at O(230 km/s). This has a few important impliciations. First, it increases the expected kinetic energy of WIMPs in our frame, thus resulting in more energetic WIMP-nucleon interactions, making direct detection (which essentially consists of having a bunch of material and waiting for an unexplained interaction within it) significantly more feasible. Secondly, it leads to an annual modulation effect (as reportedly, controversially. observed by DAMA or COGENT) related to the Earth's motion around the sun. Thirdly, it means that recoils from interactions with WIMPs will have a preferred direction away from the direction of the Earth's propagation through the galaxy. Because no potential terrestrial background can have this signature, a number of groups, including the one I work with, are trying to develop detectors sensitive to the direction of low-energy recoils in order to potentially unambiguously detect WIMPs.
