# Can Dark Matter just be clumps of Neutrons

I was wondering about Dark matter, and it occurred to me that why could it not be just nuclei of Neutrons with no electron cloud.

• Is it possible for such things to exists.
• Can Neutrons bond to one another with out Protons?
• If so wouldn't they form atom like things that can't bond with each other? Their only interaction would be through gravity, and if they collide with each other.

Its just that every star that collapses generates a lot of high density material, and if positive and negative annihilate, could it be that all that is left is just large nuclei of neutrons. Kinda like the periodic table, except with no charge, just a bunch of mass.

My understanding of quantum physics is very basic, and am keen to learn more.

• – rob Apr 14 '15 at 5:04

It can't be solo neutrons, because they are unstable and decay into protons. So far as we know, there's not a stable configuration of mostly-neutrons that occurs in nature intermediate between heavy nuclei (uranium is roughly 3-to-2 parts neutrons) and neutron stars of 1-3 solar masses (which are about 90% neutrons).

What you're describing would be the kind of dark matter called a "MACHO," or "massive, compact halo object." Thanks to recent gravitational lensing studies, where a robotic telescope continuously watches many stars to search for brightening due to the gravity of an intervening dark object focusing extra starlight on Earth, we now have a census of these items down to about the mass of Jupiter. The planet-sized MACHOs outnumber stars by about two to one, but only contribute a few parts per hundred of the total mass of our galaxy. The "dark" contribution of the mass of our galaxy is a few times larger than the luminous mass.

There's actually a pretty firm estimate of the total density of protons and neutrons (collectively, "baryons") in the universe, based on the chemistry of what's out there. Most nuclei are ordinary hydrogen; about 25% are helium-4; various tiny fractions are deuterium (heavy hydrogen), helium-3, and lithium-6 and -7. We know an awful lot about how those light nuclei interact with each other from accelerator experiments, and so we have a very convincing model of how much of each species should have been produced during the Big Bang. Furthermore we can say how many photons should have been produced per nucleus: if there were much more or less than $0.6\times10^{-9}$ baryons per photon at the time of the Big Bang, then the light-element chemistry of the interstellar medium would be measurably different than what it is.

Most sensible people are reluctant to say "the invisible stuff that makes up the bulk of the gravitating mass of the universe must be a fundamental particle that we've not encountered yet on earth." But the case for that scenario is actually quite strong.

• Ok, crazy idea here: is space-time not so distorted inside a black hole that "time stands still"? As in: wouldn't de neutrons inside the black live relatively longer when observed from outside the black hole, due to time dilation? I understand it is difficult or even impossible to talk about the lifetime of a distant object in general relativity sense, but could this idea work at all? – rubenvb Apr 14 '15 at 12:05
• Anything inside the event-horizon of a black hole is unobservable, and as such by definition outside the realm of falsifiable science. For all intents and purposes, any mass inside a black hole is just black hole mass - and the mass of black holes doesn't make up for the total dark matter mass by a long shot. – Martijn Apr 14 '15 at 13:20
• @rubenvb The lensing "census" of MACHOs includes all dark objects heavier than approximately the mass of Jupiter, including black holes. Their combined mass is less than 2% of the galaxy's luminous mass. I think the limit on planet-mass black holes may be less stringent, but there's no proposed mechanism that would produce them. – rob Apr 14 '15 at 16:51
• Thanx Rob, So Neutrons can form bonds to one another? So it could be possible for there to be clumps of neutrons forming atomic sized nuclei (maybe 2 -> 40 in size) forming clouds around the galaxies, or would we expect to see them clump into the MACHOs as you describe. – Sporky Apr 15 '15 at 7:09
• @Sporky No, there isn't stable pure-neutron matter. Look for information about the "neutron drip line" of nuclear physics. – rob Apr 15 '15 at 8:41