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[zombie killer]

i have been discussing dark matter, and i have 2 questions:
1. have we actually ever seen dark matter
2. what makes us think it is there?
if you know either of the answers, please respond!
thank you and have a nice day!

[Lectorog]

http://en.wikipedia.org/wiki/Dark_matter

[Levi]

As far as I know:

1)  No
2)  Something to do with the universe having way more mass than we can see, so we assume there must be something else there we can't see, ie dark matter. 

I don't know enough about it to say that is absolutely true, I'm sure somebody else knows more. 

[Mictlantecuhtli]

PTW

[LeoLeonardoIII]

I'm just summing up the wiki article here, but we know DM exists because there's something out there with a lot of the properties of matter but we can't see it - it doesn't respond to light the way we expect. We might have our calculations wrong, but we're pretty darn sure it's right.

One type of DM is the neutrino, which interacts with other matter only occasionally. Right now you have an enormous number of neutrinos whistling along through your body, fired away from the Sun all the time, but they almost never actually touch the matter that makes you up. (EDIT: Wikipedia says 65 billion neutrinos per square centimeter of your body per second, 24/7, because they also pass through the Earth to you at night). Neutrinos are very light, but we expect to find particles that are individually heavier at some point. We expect that the heavier DM is doing the same thing but we haven't noticed it yet. We're currently looking!

You wouldn't fly through space and suddenly run aground on a big black rock of DM. It's more like how children don't recognize that there are governments out there, but the governments are there and are affecting the child's life. We're growing up, and learning.

[wierd]

There are 2 prevailing theories for dark matter candidates.

"WIMPs", and "MACHOs",

Or, Weaking Interacting Massive Particles, and Massive Compact Halo Objects.

Wimps would be things like neutrinos, or neutron radiation clouds.
Machos would be something more like dense clouds of matter of an unknown type that concentrates in the halo around galaxies.

There are some on-going experiments to attempt to measure the ambient neutrino flux in space away from terrestrial sources of them, to see if neutrinos can account for the extra mass observed, but so far neutrino flux alone isn't able to explain things like the bullet cluster.

IIRC, a resent meta-analyis study of satellite tellemetry patterns suggest that the earth has a halo of dark matter as well, but I don't have a link handy.

Essentially though "dark matter" is an apellation applied to a phenomenon that has been observed distantly in places like the previously mentioned bullet cluster, but also has pronouned and measurable influence on our own milky way galaxy's shape and rotational properties. The observable matter in both locations is insufficient to explain the kinds of rotational and gravitational system effects we are observing. "Dark matter" is a kind of place holder to explain the incongruity. At the moment, nobody really knows for certain exactly what it is, but we can infer some of its properties.  It appears to not interact with light, so it must be electromagnetically neutral, and it clearly has mass. That's about all we do know.

[Steeled]

Last time I checked, we do know what dark matter looks like.

Spoiler (click to show/hide)

[LeoLeonardoIII]

Not quite. Maybe a bit darker? I see some light interacting with it. I can tell because I have seen a lot of matter in my day and because of some of the pixels.

[wierd]

Here's a link to a new scientist article about the satellite data meta-study I mentioned.

If correct, it means there is dark matter very close to home that can be directly studied.
Sadly, most of our observational testing aparatus relies on electromagnetic radiation based interactions, which dark matter won't respond to.

Still a tantalyzingly interesting proposition all the same however.

[Putnam]

We know nothing about Dark Matter except that it has mass and does not interact electromagnetically. Dark Matter is the simplest and thus the best explanation of the formation of galaxies as we know them; its existence is simply the easiest way to describe why that is. It more than likely exists, but we have no idea what form it takes if so.

[Squill]

Not sure if I'm right, but I believe I heard a theory that the expansion of the universe (ie. the growing distance between various celestial bodies) was accelerating, rather than decelerating from gravity, and the theory was that dark matter was somehow "pushing" the parts of the universe away. So if there are two galaxies that shot directly away from each other at the beginning of the universe, rather than being slowed down by gravity and eventually completely reverse course and come crashing back together, they would constantly accelerate faster away from each other.

FAKE  EDIT: According to Wikipedia, it is dark energy rather than dark matter. (Matter is to energy as dark matter is to dark energy.)
Also, as further confusion, theoretically there is dark matter, antimatter, and negative matter, all of which are different things.

[wierd]

Well.. if you can manage to send a probe to a black hole in any sensible timeframe to experimentally verify the existence of hawking radiation, you would by necessity show that antimass exists, since hawking radiation works through virtual antiparticle pairs interacting with the black hole's event horizon, where the particle being captured by the event horizon has antimass.

The existence of antimass wuld have very significant consequences.

[MetalSlimeHunt]

But if mass is the property of not being light, would antimass be the reversed property of not being light, or would it be antilight?

[MetalSlimeHunt]

I wouldn't exactly describe annihilation as "poof". It is significantly more violent than that.

[wierd]

Photon is its own antiparticle. Photons interact with positive biased particles just as well as with negatively biased ones. It does not interact with electromagneticaly neutral particles.

Antimass is hard to explain.

If you take the "rubber sheet" analogy, normal mass makes a dimple. Antimass makes a hill.

The higgs field interaction idea, states that the properties of massed particles causes higgs and virtual higgs particles in the scalar higgs energy metric to cluster around the object, making it 'sticky'. Antimass would actively repell higgs particles and virtual higgs particles. To date, no particle with antimass properties has been detected. At least none that I am aware of.

However, hawking radiation comes from the ambient quantum foam instability of spacetime itself interacting with the event horizon, which is jut a way of saying that spacetime is lumpy at that scale, and that the balance between the bumps and hills is a zero sum. (That's why it appears more or less smooth on the macro scale) at the black hole, the bumpy parts fall in, and the dimply parts fly away. With the equlibrium disturbed, the particles are pushed on shell, and made "real". The energy needed for this comes from the black hole. This makes the black hole shrink.