Is dark matter all around us?

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Summary:

Is dark matter all around us?

Main Question or Discussion Point

I understand black matter surrounds galaxies and galaxy clusters, but is it inside the galaxies themselves? Is it in interstellar space? If so, does it surround the sun and planets, and if so, could it be right here on Earth with us?
 

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  • #2
ZapperZ
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Black matter? Or do you mean dark matter?

Zz.
 
  • #3
Drakkith
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Yes, dark matter is believed to be inside our solar system and our galaxy, but only in very small concentrations. Most of the nearby dark matter is contained in a 'halo' surrounding the galaxy.
 
  • #4
phinds
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It is estimated that the total mass of the dark matter in our solar system is approximately that of a fairly small asteroid.
 
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Yes, dark matter. Sorry
 
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Vanadium 50
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Black matter? Or do you mean dark matter?
Given that he knew it was called Dark Matter back in April, it's unclear what he means.
 
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zuz
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But could it be here on Earth?
 
  • #8
phinds
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But could it be here on Earth?
Of course it's here on Earth, but in utterly trivial amounts. There are great efforts underway to detect it.

EDIT: as is clear from subsequent posts, I should have said "passing through" Earth, not "on" Earth.
 
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  • #9
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Yes, dark matter. Sorry
If you‘re not a native English speaker we’ll forgive you :smile: and we’ve corrected the title.
 
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Dark matter should cross Earth easily and without interaction in nearly all cases. Physicists look for the extremely rare non-gravitational interactions (if they exist) in dedicated detectors.

XENON1T has seem some unexplained events - most likely a more mundane explanation, but if we are really lucky it's a first sign of dark matter detection in the lab.
 
  • #11
DaveC426913
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But could it be here on Earth?
"On" Earth is a bit misleading.

One of the characteristics of DM is that it doesn't interact with normal matter, so it's not going to be sitting around on the ground. It doesn't get stopped by Earth.

Indeed, it won't be sitting anywhere at all. DM particles are likely to have a large velocity relative to Earth - since they don't interact with matter, there's nothing to slow them down. Meaning any particles we might try to detect will be moving at a non-trivial velocity. We're being bombarded.
 
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ohwilleke
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It is estimated that the total mass of the dark matter in our solar system is approximately that of a fairly small asteroid.
To embellish on this point a bit, the key fact necessary to understand how something so diffuse (assuming it exists and is not really some 5th force or modified gravity effect) is that there is an almost unfathomable amount of almost completely empty vacuum in the galaxy and its vicinity. Quoting from another PF post (paragraph breaks added for ease of reading):
The volume of the solar system inside Pluto's orbit is ~10^39 m^3, while the volume of the galaxy inside a radius of 10 kpc is ~10^62 m^3.
Assume for the sake of argument that the density of dark matter in the galaxy is ~.01 Msun/pc^3. It isn't a constant density, but this will illustrate the point.
Then the total mass of dark matter inside Pluto's orbit is ~10^-13 Msun, which is completely negligible.
On the other hand, the total mass of dark matter inside a radius of 10 kpc is >10^10 Msun, which is comparable to the mass of stars.
From here.

The Milky Way has about 400 billion (i.e. 4*10^11) stars.

The Earth has a mass ca. 3*10^-6 MSun, and the Moon has a mass ca. 3*10^-8 MSun.

Thus, the total mass of all of the dark matter inside Pluto's orbit, if it exists, is on the order of about 1/100,000th the mass of the Moon (or a Moon rock material body with a radius of about 100 miles which is about the size of a big asteroid), but distributed somewhat evenly across the entire solar system.

Since people feel only the net pull of gravity from all directions and the force of gravity declines as 1/r^2, only the somewhat local inhomogeneities in a distribution of dark matter are observable to someone inside that distribution of dark matter.

So, the gravitational impact of dark matter in the vicinity of the solar system on someone in the solar system is much, much smaller than the gravitational impact of a single large asteroid within the solar system.

The map below from xkcd of the relative surface area of planets, moons and asteroids with solid surfaces in the solar system can also help put in perspective how small a single asteroid is (all asteroids in the solar system collectively have the surface are shown to the top right of Siberia and one with the presumed mass of all dark matter in the solar systems would be one among a vast number of them):

http%3A%2F%2Fimgs.xkcd.com%2Fcomics%2Fsurface_area.jpg
 
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  • #13
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Since people feel only the net pull of gravity from all directions and the force of gravity declines as 1/r^2, only the somewhat local inhomogeneities in a distribution of dark matter are observable to someone inside that distribution of dark matter.
Even a homogeneous distribution can be measured (in principle). Just like you have tidal gravity within Earth (or an idealized homogeneous Earth) you get tidal gravity within a uniform dark matter distribution. If we could map all regular matter in the Solar System well enough and account for tidal forces from other sources we could look for dark matter in the Solar System.
There are also arguments that the local density should be much higher from dark matter captured by Solar System objects. They estimate 1020 kg caught, 1/700 the mass of the Moon, or 40% the mass of Vesta.
 
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ohwilleke
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Even a homogeneous distribution can be measured (in principle). Just like you have tidal gravity within Earth (or an idealized homogeneous Earth) you get tidal gravity within a uniform dark matter distribution. If we could map all regular matter in the Solar System well enough and account for tidal forces from other sources we could look for dark matter in the Solar System.
There are also arguments that the local density should be much higher from dark matter captured by Solar System objects. They estimate 1020 kg caught, 1/700 the mass of the Moon, or 40% the mass of Vesta.
Good catch. Of course, the level of precision in measurement necessary to actually do that is far greater than what exists today.
 

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