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Comment by Cacti | Hacker News Reader

Dark matter is largely an instrumentation problem. The universe is very big, and direct observation of individual objects is largely limited by distance/luminosity, which is problematic when you are looking for lots of old dark stars very far away. The history of astronomy is largely a process of deriving estimates of new phenomena from what we can observe, and thus underestimating the significance until a better instrument comes along (as at first we only can see the larger objects, which often results in undercounting).

Dark energy is partly instrumental, but largely a theoretical gap in understanding. God only knows what is going on there.

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The problem of dark matter isn't best thought of as 'dark matter is theorized as an explanation for observations'. It's better thought of as 'observations of the mass of matter in the universe are inconsistent with observation of the amount of light coming from matter in the universe'.

Matter generally does two things: interacts with other matter through gravity, because it has mass; and emits EM radiation, because it has temperature.

When you look at galaxies and try to figure out how much matter they contain, if you look at the gravity, you get one number for mass that implies one quantity of matter; and if you look at the EM radiation, you get a smaller number that implies a lower quantity of matter.

So the conclusion is, there must be some matter that's causing the gravitational effects, but that's not emitting any EM radiation. Matter that is dark. Dark matter.

I blame George Lucas for a lot of this confusion. When people hear 'dark matter' they think it's dark in the sense of 'mysterious'. It's way more literal than that. It's just matter that's dark.

So it's cold matter, basically? Matter that's at or close to absolute zero temp?

I mean, it could be - but you'd need to come up with an explanation for why it isn't warming up at all. Most matter reaches an equilibrium temperature where the outgoing EM radiation equals the incoming EM radiation. This matter isn't doing that.

There are areas of the universe that are "voids" where there's almost nothing, maybe that's where this cold/dark matter is? There's nothing nearby to warm it up? Though this article seems to be specifically about the Milky Way.

Most of the study around dark matter is in the context of galaxies because that's where the gravity/light mismatch occurs. The rotation of galaxies (as influenced by the gravity of the matter in them) indicates there's more matter than what we can see via the light they emit.

That mismatch when observing galaxies is the whole reason we think dark matter is a thing.

Most of the evidence is localized around galaxies (not just rotation curves, but also radial velocities and excess gravitational lensing), but not all. Even uniformly distributed dark matter would still show up as a contribution to the mass density of the universe, which curvature measurements indicate is much higher than baryonic matter alone can account for.

There are potential dark matter candidates are hot like new kind of neutrinos. But observations have ruled those out so the remaining candidates are mostly cold.

Most of the dark matter candidates are not ordinary matter but particles that don’t interact with ordinary matter.

"Cold" in this context means "a typical particle is moving slow enough that we can safely neglect relativistic effects". Hydrogen plasma on the surface of the sun is "cold", in this sense. Solar neutrinos are not.

This is unlikely. There are many many theories of how dark matter could be accounted for with ordinary matter that we simply can't detect because our telescopes/etc aren't good enough.

Not all of these theories are completely excluded yet but most have very very thin margins of phase space left to explore (even when combining multiple explanations together). Every time a new telescope comes online we see the phase space diminish rather than hints towards first observations.

We are left with:

A) new particles that don't (or very weakly) interact with electro magnetic fields.

B) New theories of gravity.

C) New theories of the early universe that open up phase space previously thought closed to existing matter contributions

D) better instrumentation that sees actual contribution in the tiny phase space left to ordinary matter and ordinary physics

D is by far the least interesting of these options and so gets very little press. But it gets plenty of academic attention and you can be assured it is not being ignored by scientists

> Dark matter is largely an instrumentation problem

The bullet cluster refutes this hypothesis [1].

[1] https://en.m.wikipedia.org/wiki/Bullet_Cluster

No, the current most reasonable explanation for the dynamics of the bullet cluster are merely consistent with our current best dark matter theories. It is suggestive at best.

I’m not claiming those observations prove dark matter. I’m saying they are problematic for the claim that this is an instrumentation issue.

That's not what 'Dark' means in this context. It's not that it is too far away to be detected, it is non-luminous (as in: not emitting any electromagnetic radiation) and so it isn't detectable other than by its secondary effects on other objects.

Black holes are another example of something that we can not directly observe using instruments, but that we can observe through their secondary effects. But black holes are part of the cosmic accounting book in an identified manner, dark matter is not.

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