Dark Matter and Dark Energy

[Ganymede12]

I have heard these two terms pop up in many documentary's and no matter how they explain it I still struggle to wrap my head around them.

So what is the most simple definition of Dark Matter?

What Is the simplest definition of Dark Energy?

And what id the difference between them?

I asked my physics teacher but it's been weeks and I still haven't got an answer!

[SionR25]

Dark matter is a hypothetical solution that makes up for what we believe is missing mass in the universe. Physicists think there is mass missing withing the universe and dark matter is believed to be that missing mass, although it can't be seen and makes no visible effect that we would notice.

Dark energy, from what I understand, accounts for the expansion of the universe. It is an energy found in space that has the opposite effect that gravity has, it forces objects away from each other. This again is a hypothetical solution and something we can't/haven't yet been able to see.

HTH,

Sion

EDIT: Someone feel free to collect, this is just what I believe from reading stuff and I have never been taught any of it . Also, hope it isn't too brief

[Ganymede12]

Why do scientist think there is missing mass in the universe? Did it put up a missing poster?

[Treeden]

Grunthos posted a link in the Universe thread that provides an interesting answer, or possible answer.

[SionR25]

Why do scientist think there is missing mass in the universe? Did it put up a missing poster?

They think there is missing mass because the results from their gravitational field strength and the mass of what they are made from (E.G. stars being Hydrogen and Helium) does not match up. By creating this 'Dark Matter', which is about 84% of the mass of these bodies, the values make sense.

Sion

[sologuitarist61]

It is one of those strange things that you find now and again. A theory put forward because of observed data. As far as I am aware neither dark energy or dark matter has ever been discovered/observed - just their effects on the universe.

[JulianO]

Just to chime in.

To the original question, about the only thing the two have in common is the word dark - and in a sense the dark is there in both cases because we don't really understand what it is.

Dark matter is something a bit like normal matter, but it doesn't interact with electromagnetism, so it doesn't shine, and light passes straight through it - so you can't see it. It has mass though, and so has gravity associated with it, and it fixes a whole bunch of issues seen observationally. It's also probably not dark, as it can interact very weakly with normal matter, and with its self. So there are a number of cases of people looking for its signature in the heart of galaxies - which would make it not quite as dark as firs thought - maybe grey! There are some good candidates for the possible particles it is made up from, but so far they have elluded detection (but they are very hard to detect!)

Dark energy is a repulsive force that seems to be a property of space. The bigger the space, the more this repulsive force comes into existance. As the universe expands, this force being dependant on the amount of space is growing, so the universe is accelerating apart. There are a few theories kicking around for what Dark Energy may come from, but nothing that is solid as yet.

So Dark Matter we understand fairly well, we can model it, build galaxies with it and so on. We're still not quite sure what it is.

Dark Energy is a bit behind Dark Matter in terms of understanding - but hey, we have a name for it and thats a start!

[Ronnie67]

No one knows what dark matter is, but it constitutes 80 percent of the matter in our universe, but by studying numerous dwarf galaxies - satelite systems that orbit our own milky Way Galaxy, Nasa's Fermi gamma ray space telescope has produced some of the strongest liimits yet on the nature of the hypothetiucal particles suspected of making up dark matter. .

[Ganymede12]

So in short.....

Dark matter is invisible mass that that gives an object more gravity

And Dark energy is a mysterious force that is pushing everything in the universe away from each other and making the universe expand?

[JulianO]

Dark matter is invisible mass that that gives an object more gravity

And Dark energy is a mysterious force that is pushing everything in the universe away from each other and making the universe expand?

Pretty much, except it doesn't really give an object more mass, its just extra mass that is lying around in the right places so when we weigh galaxies it comes out right. Typically the dark matter halo is about 10x the visible size, so if you could see DM, our galaxy would look something more like this.

[Ganymede12]

So dark matter is additional matter that, so far, is completely theoretical and can't be detected?

[sologuitarist61]

If dark matter has mass and therefore gravity, then shouldn't we be able to see it wherever we look through gravitation lensing? :huh:

[Canholder]

Hi sologuitarist61, Yep from what i understand that has been tried and the dark matter appears to form a halo around the galaxies.

i always believed the main evidence for dark matter come from the rotation of galaxies, The outer stars orbit too fast about the centre to stay in orbit given the amount of matter observed in the galaxies.

Paul

[JulianO]

So dark matter is additional matter that, so far, is completely theoretical and can't be detected?

Yes - a bit like we were with the neutrino a few decades back. No detection, but its the simplest answer that fits all the facts. There are other options.

If dark matter has mass and therefore gravity, then shouldn't we be able to see it wherever we look through gravitation lensing? :huh:

Indeed we can - which leads onto...

Hi sologuitarist61, Yep from what i understand that has been tried and the dark matter appears to form a halo around the galaxies.

i always believed the main evidence for dark matter come from the rotation of galaxies, The outer stars orbit too fast about the centre to stay in orbit given the amount of matter observed in the galaxies.

Rotation was one of the first clues that something was wrong. However lensing shows that there is again more mass present (the light is bent more than it should based on visible matter) and not just a little bit, but significant quantities, and again squares with dark matter being present. There are other lines of evidence such as signatures in CMB radiation, and big bang equations that predict how much matter should be present.