Dark Matter

[godBoy]

I recently attended a public astronomy seminar that discussed Dark Matter,
An invisible halo of invisible matter that surrounds our galaxy(and others). It is said to make up 90% of the mass of our galaxy.. But then Dark Matter and the visible matter are said to only make up 26% of the total energy density. Dark Energy makes up the other 74%..

What is the deal with Dark Energy? What is energy density and why can't there be nothing in the space of our unseen galaxy?

[cambraceres]

Dark Energy is a way to use relativistic arguments to explain away certain astronomical anomalies. The universal expansion for instance. Energy Density is self explanatory, but there is one thing you may be missing.

There are two different Dark Energy models, one which assumes a homogenous energy distribution, and consequently, uniform energy density, and the other considers space to be "filled" with differing amounts of energy in seperated regions.

I don't want to get into the obvious issue, just letting you know about a possible lapse in your understanding.

Can you clarify this?

[ QUOTE ]
why can't there be nothing in the space of our unseen galaxy?

[/ QUOTE ]

Is "unseen" part of a more subtle question or are you asking why space can't be empty?

Cam

[MidGe]

[ QUOTE ]
... or are you asking why space can't be empty?

[/ QUOTE ]

Because nature abhors a vacuum. /images/graemlins/smile.gif

[Metric]

There could be zero dark energy -- in fact this was the default expectation ever since Hubble discovered the expansion of the universe. It just turns out that recent measurements seem to imply a non-zero dark energy density that is 122 orders of magnitude off from what straightforward theoretical calculations seem to tell us to expect. So obviously there is a big problem here. Some radical new way to rethink the problem is needed.

[godBoy]

I thought the images from WMAP showed pretty conclusively that there are "differing amounts of energy in seperated regions"

And yes, why can't space be empty?

[Andrew Karpinski]

godBoy : I believe I can answer your last question. I am going to research it a bit, but the answer is (I think) that their are equations which calculate the amount of matter in the galaxy, and the visible matter does not meet the amount in the calculations.

[Andrew Karpinski]

this may help (http://www.astro.queensu.ca/~dursi/dm-tutorial/dm0.html)