What is dark matter, and is it proven to exist?

[flobadob]

Could someone explain in lay terms what dark matter is and also is there any evidence of dark matter's existence or is it a theory?

[Pigletion]

In simple (hopefully) terms

Dark matter is the 'stuff' that is believed to exist, but which can not be seen, which explains the gravitational force exhibited as you move further away from a galaxy.

When plotting a graph of rotational velocity versus distance from the centre of a galaxy, the rotational velocity should decrease with distance, as the gravitational pull would decrease the further away from the mass of the galaxy you got (from standard laws of physics).

However, this does not happen. The graphs flatten out. Hence, it was postulated that there must be some mass occupying these areas that can not be seen, but that still has velocity, i.e. dark matter.

I don't believe that it's existence has definitively been proven (well it hadn't when I finished my physics degree), but there are experiments to find the particles that are believed to constitute it, e.g. WIMPS (weak interacting massive particles). Experiments were being carried out underground in a mine in Yorkshire to try to identify these particles.

[Symmetryigr8]

Its not all wimps, can be anything as long as it doesn't radiate light and no light bouncing off it has reached us.

Whole asteroid fields for example

[Pigletion]

Yes agree, not exclusively WIMPS, that was just an example.

Just thought that may be a starting point for further research, particularly when Googling etc.

There used to be a web site by the UK Dark Matter collaboration that my lecturer was a member of (that said about the Boulby mine experiment, amongst others), but they seem to no longer exist. Maybe they got rebranded!

[schonovic]

i ran into a theory that dark matter was large quantities of long chain carbon molecules in interstellar space. i don't know if that has been proven or disproven.

[R1Geezer]

It is matter that is detectable by it's gravitational effects, but it is not in the form of atoms that we understand. Don't know what symetryIgr8 is on about, asteroid field are not dark matter, and lt's got naff all to do with light reaching us.

Dark matter us essentially the name given to some form of matter that must exist or Galaxies could not coalesce but we cannot detect it directly, it is essentially a mathmatical device to make the equations work. Only 5% of the matter in the universe is in the form of Atoms as we know it, the rest is darkmatter and dark energy.

[flobadob]

So it basically something that no one can see but they just believe in. Interesting. What does that remind me of?

[jake-the-peg]

No No No

We do know it's there because we see it's gravitational effect.

You don't have to see something to know that it's there! Close your eyes and stick your fingers in the mains!

It's a question of what it is.

There are 2 schools of thought Wimps and Machos

The wimps Pigleton has described ( sub atomic particles possibly new but possibly neutrinos )

The Machos (MAssive Compact Halo Objects) SymetryGirl menions burnt out stars etc.

Try here http://www.bbc.co.uk/science/space/deepspace/d arkmatter/

[Pigletion]

No, there is evidence for it, but not visual or tangible (i.e. we can't touch it). That's why it was postulated in the first place. Why else are there gravitational effects? To have gravitational effects you need mass (ask Mr. Newton), so there must be some there. This is based on facts from experiment, not fantasy.

[flobadob]

Question. What holds an atom together?

[R1Geezer]

strong nuclear force

[Pigletion]

strong nuclear force holds the nucleus togther, electrostatic force keeps the electrons orbiting, particles themselves are made of quarks held together by gluons.

Put simply, there are four forces, the weak and strong nuclear forces, the electromagnetic (includes electrostatic) and gravitional. The aim is for a Grand Unified Theory to unite them or The Theory of Everything as it is sometimes called.

This stuff is easy to find on the net (if you wish to know more), in Physics textbooks or A Brief History of Time by Stephen Hawking is a good read.

[Symmetryigr8]

We can't touch most things in space (only the moon really).
So far at least...

[flobadob]

What I was wondering was could this "dark matter", that apparently is holding the universe together, be the same stuff as that which holds atoms together?

[jake-the-peg]

No,

It's not holding the universe together, it's just keeping galaxis of stars from seperating. Without it the stars and planets would just spread out more.

It just exerts plain old common or garden gravity.

Atoms are held together by electromagnetism and nucelii in atoms by the Strong force as already said.

Here's a thought tough - Neutrinos are tiny particles - we thought they had no mass but we now know they do but it's tiny, so tiny nobody has been able to measure it yet. We just know it's there from their behaviour.

But there are a lot of neutrinos - the sun alone sends 65 billion per second per square centimeter through the Earth.

That adds up!

There are reasons why neutrinos may not be the whole answer but they are very interesting

[Rev. Green]

Peter Rowlands suggests (Zero to Infinity p606) that the effect of dark matter could be caused, instead, by an inertial event horizon. He cites as evidence the fact that the ratio of dark matter to visible matter would have to increase dramatically with the size of object, from stars, to galaxies, to galactic clusters and superclusters.

(On the previous page he also dismisses dark energy: showing that the mass of the vacuum is two-thirds of the total mass of the Universe.)