# Dark Energy & Dark Matter [closed]

Do we know so little about dark energy and dark matter that it could be all around us and maybe even within our own bodies?

## closed as unclear what you're asking by John Rennie, ACuriousMind♦, Diracology, user36790, sammy gerbilJul 26 '16 at 16:38

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• Yes. Dark energy is generically uniform, so it would be suffusing us. We don't know what dark matter is, so the answer depends there. If it's WIMPs, for example, dark matter particles would likely be constantly going through us, like neutrinos, but not doing anything. – knzhou Jul 26 '16 at 2:33
• @knzhou: We don't know, yet, how uniform dark energy really is. The statistics of currently available measurements is completely insufficient, but I wouldn't be surprised to wake up one day and the science news says that someone has measured potential inhomogeneities on the largest scale of the universe. – CuriousOne Jul 26 '16 at 3:32
• @CuriousOne. Maybe, but the homogeneity and isotropy of the CMB and the expansion does say something about the dark energy density constancy over space and time. Not just one number, if it varied much it'd be detectable in the observations of CMB and the acceleration. – Bob Bee Jul 26 '16 at 5:42
• @BobBee: All you need to do to see cosmology that was dead wrong is to read "The first three minutes" by Weinberg. It's an excellent book. I love it. It was written before precision cosmology was mature and it made the best of the material that it had... the material just wasn't good enough. That's all I am saying... take the conservative approach to data, which means that you can't rule anything out that is beyond its precision limitations. – CuriousOne Jul 26 '16 at 5:52

In the current paradigm of cold dark matter, then yes, dark matter is everywhere in space, and can pass through and inside your body. It must be so weakly interacting with normal matter that there is nothing to stop it. However, the answer to whether there is any dark matter inside you now turns out to depend strongly on what that dark matter is.

Dark matter is not distributed uniformly throughout the universe - it should be denser towards the centres of galaxies and there is dynamical evidence for this. Dark matter is expected to be fairly uniformly distributed over solar system scales, though there should be small effects due to gravitational focusing. The best estimates of the dark matter density in the disk of the Galaxy is $\sim 10^{-22}$ kg/m$^3$.

If dark matter were made of weakly interacting massive particles (WIMPS) with rest mass energies of $\sim 100$ GeV (as is commonly supposed), then there is only one of these particles in every $\sim 10^{7}$ cubic metres of space! i.e. $n_{\rm WIMP} \sim 10^{-7}$ m$^{-3}$. Your body has a volume of around 0.1 m$^3$, so there would only be a $10^{-8}$ chance of your body containing such a WIMP at any instant.

But the solar system is travelling though a "wind" of dark matter at $\sim 200$ km/s, so if your cross-sectional area is $\sigma \sim 1$ m$^2$, your mean free path is $(n_{\rm WIMP} \sigma)^{-1} \sim 10^7$m, and so you will encounter a 100 GeV WIMP every $\sim 50$ seconds.

You can repeat these calculations for lighter dark matter candidates. The lightest candidates - axions - may have rest mass energies as low as $10^{-5}$ eV. There could be $10^{17}$ of these in your body right now.

Dark energy is a response of the GRT followers to that they can't explain the shape of galaxies.

The Heaviside style GEM does indeed give the correct shape without adding a fix to nature to make the equations work.