## General background

A lot of ink and electrons have been spilled over the question of what dark matter is. A theory about it has been kicking around in my head for a few years now. It could be completely wrong. But I haven't seen it out there before, and it doesn't obviously fly in the face of observations, so I'm putting it out there. If it's wrong, so be it.
I think most of my readers are already familiar with dark matter, so I'm just linking to the wiki entry. But I do want to highlight a few points:
• Whatever dark matter is, it's probably not an undifferentiated swarm of particles (WIMPs usually). This is known from the bullet cluster (1E0657-556) and similar observations.
• There's not enough gravitational lensing for it to be mostly or all MACHOs.
• And despite some neat fits to observations, theories like MOND that modify gravity have extreme theoretical difficulties.
• While dark matter is generally co-located with visible galaxies, there are seemingly random exceptions in both directions. Eg, NGC 4736 and VIRGOHI21.

## Parallel Dark Matter

Probably the easiest way to explain Parallel Dark Matter is to start by pointing at braneworld cosmology. That doesn't mean PDM is committed to Braneworld cosmology, just that they fit neatly.
Braneworld cosmology says that everything we see, everything in the visible universe, is actually stuck to the surface of a brane. The only things that aren't stuck to it are gravitons, and that's used to explain why gravity is so weak. One braneworld theory, the Ekpyrotic universe, proposes two branes which collided and more-or-less made the Big Bang.
PDM proposes that there are 6 branes including ours, all about equally full. It proposes that dark matter is the matter of other branes, whose gravity alone escapes those branes.

## A few immediate objections

### But branes+gravitons is more complex than that

Q: Gravitons of low energy might not escape branes, or might not get back in (it might polarize a brane and bounce off). AFAIK, we can't measure that. So 5 hidden branes might actually not give the 5:1 gravitational pull that we see.
A: All true. The simplest form of PDM ignores that, so it gives just a first approximation. To a second approximation, it might be one of the following:
• The required energy to escape/enter is so low as to be negligible.
• It's not negligible and there are more than 6 branes.
• The substrate is something other than branes.
If PDM comes to seem likely, it might let us see which is the case and provide a means for estimating the escape and entry energy.

### So where's parallel earth?

Q: Where are the 5 parallel earths?
A: There aren't any. PDM does not propose that the other branes have parallel structure in their details. The neighborhood of our solar system is likely to be about as empty of dark matter as some randomly chosen equal-sized region of interstellar space.

### How is it a halo?

Q: Why hasn't the dark matter collapsed into a galactic plane? Why is it still a spherical halo?
A: It will sometimes have collapsed individually for each brane but still in aggregate appear as a halo. Like a honeycomb paper party ball, each of its planes is flat, while the whole still approximates a sphere.

Of course the illustration overstates the case. It has a few dozen planes instead of 5 (dark), and their alignment is regular and efficiently makes an approximate sphere. In PDM the orientation of galactic planes should be random, so it doesn't make a sphere as efficiently.
Dark galaxies would probably be concentric with each other and with the associated visible galaxy, but random in orientation.

### But that's MACHOs!

Q: But MACHOs were ruled out by lensing observations. That would rule out "parallel brane" MACHOs too.
A: The non-dark universe is mostly not massive compact objects. It's mostly neutrinos, and a lot of the rest is gas. And the MACHO observations leave room for a fair fraction of dark matter to be MACHOs, just not all or most of it.

## How could it be tested?

Some predictions of PDM that seem testable:
• In a small percentage of spiral galaxies, the dark matter should be not a halo but a disk. I said earlier that, for dark spiral galaxies, their alignment is random with respect to our own and each other. So sometimes by pure chance they will all nearly line up.
• Further prediction: such disks will have no particular orientation with respect to the visible galaxy's disk.
• Unseen stellar partners should be more common closer to the galatic center. That's because dark objects should be gravitationally capturable just like visible objects. However, they would tend to move relatively faster with respect to visible objects, because they tend to be orbiting in a different plane, so they're still individually less likely to be captured than visible objects.
• The incidence of unseen stellar partners, in proportion to captured visible stellar partners, should be noticeably higher inside the galactic bulge, where all the dark companion galaxies overlap ours.
• It should not vary so much within the galactic bulge.
• It should have a very fuzzy boundary due to drift and to inexact overlap.
• Unfortunately I've lost the reference for this, so it's going to be vague. I figured I'd still put it out there as a possible disconfirmation. A few years back, astronomers found a flow of dark matter in our own galaxy. If PDM is true, it seems likely that such a flow would be part of a dark galaxy galactic plane. It should be approximately in a plane that goes thru the center of our galaxy.
• There would be other dark flows in the same direction along the same galatic radius, within a few degrees to account for the dark disk's thickness.
• There would be dark flows in the opposite direction along the opposite galatic radius, again within a few degrees.

## General disclaimer

This is just a hypothesis I'm putting out there. If it's wrong, so be it.

1. This reminds me a bit of Roger Penrose's suggestion that (if I got the gist of it right) the universe has gravity waves in it that are surviving echos from pre-big-bang events. For a poor slob like me, trying to get some handle on this stuff from very general principles, the question becomes which fares better with Occam's Razor: echos from previous incarnations of the universe, or crosstalk from parallel universes.

I'm inclined to suspect that the whole infrastructure of modern physics is seriously mistaken about something so basic that nobody is even looking at it. Not just quantum mechanics (though I love to hate that), but relativity too, and the faulty assumption should also be shared by classical physics. (That should narrow it down: an assumption that all three of those theories have in common. Doesn't leave much to choose from.) That's not (just) a contrarian view. The state of modern physics reminds me strongly of the phenomenon, from my studies of abstraction, in which one makes a seriously unfortunate decision in setting up the basic structure of one's system, and then later, unwilling or unable to revise the basic design, adds on kludges trying to make the poor early choice work well enough to go on with. And of course the more one invests in the kludges, the more one wants to hang on to them.

2. Oh, definitely. "The question is not is it weird?' but is it weird enough?'" - but it's really which basic assumptions are misleading us. That's what attracts me to Smolin's natural selection of universes and to Loop Quantum Gravity. Which challenge respectively the assumptions that universal constants are furthermore absolute constants and that 3-dimensionality is fundamental (or that N-dimensionality with 3 large dimensions is).

I'm not doing anything so basic, though. PDM is just an idea I've been mulling over to explain the mystery of dark matter.

3. My image link was messed up; fixed.