Pithy answer - dark matter, whatever it might be (WIMPs or otherwise) is most importantly, Weakly Interacting (the WI in WIMP). It appears to clump around gravitational structures, acting like normal matter, but in more of a field way, rather than a matter way (think of it like a fluid filling a container, with uneven density - where the density is higher is where you've got matter pulling at it).

For dark matter to be causing this, you'd need some exotic form of something that's already exotic - it'd have to be able to interact with an intensity we don't associate with it. It'd also be defying what we understand it to normally do (you'd need something to make it pull into a lump from how we think it behaves, which predisposes a physical structure like a planet).

So, is there some massive errant lump of dark matter pulling the solar system around? From what we infer about how dark matter behaves from observation, no.

(The fluid thing is not an entirely accurate model of how dark matter works, but it's a decent enough metaphor. However, this gets very complex and I'd rather not spend the rest of the day getting into ever more detail about theoretical physics.)

>"(you'd need something to make it pull into a lump from how we think it behaves, which predisposes a physical structure like a planet).

So, is there some massive errant lump of dark matter pulling the solar system around? From what we infer about how dark matter behaves from observation, no."

Apparently people find it plausible that dark matter consists of primordial black holes:

"An intriguing alternative view is that dark matter is made of black holes formed during the first second of our universe's existence, known as primordial black holes. Now a scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, suggests that this interpretation aligns with our knowledge of cosmic infrared and X-ray background glows and may explain the unexpectedly high masses of merging black holes detected last year." http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg...

So I don't see how the "clumped/lumped" dark matter explanation can be dismissed, black holes are basically lumps of pure mass right?

From what we think atm, you can't get a lump of dark matter, without having something to draw it in. It seems to gather where space-time is already distorted. Although it doesn't seem to need much (some galaxies are "made" mostly of dark matter - i.e., they get most of their gravity from it, rather than from the matter we observe), it does look like dark matter only clumps around spacetime distortions (gravity wells basically), rather than being able to clump on its own.

So yes, you could in theory have a black hole which has dark matter around it, which could do this, but it'd be mostly the black hole, not the dark matter doing the pulling.

Hope that made some sense!

>"you could in theory have a black hole which has dark matter around it"

Well, the link I provided gave me the impression it is plausible that the black holes are dark matter. This sounds like a different idea than what you are rejecting as "possible but unlikely".

Afraid that's not the case. Black holes are categorically not dark matter.

Well, ok. But apparently someone at NASA thinks it is plausible. Argument from authority heuristic tells me to listen to them rather than you in this case. Since you are not coming with any links or specific argument against what was said there, that is pretty much all I have to go on.

Primordial black holes are the candidate here as you mentioned (black holes exist, primordial black holes may exist, but we haven't found any yet).

Thanks to LIGO we think we may have the first evidence for them, but there is still a lot of work to do to prove them.

Decent article on that: https://www.sciencenews.org/article/ligo-black-holes-dark-ma...

The interesting thing will be if we can prove they are around, which would then give some options for showing if they're dark matter or not (looking for gravity waves in areas where there's large dark matter concentrations would be the most likely candidate for now).

My understanding of current mainstream thought is that gravity due to a black hole of a given mass will have the exact same effect as from a planet of the same mass.

Not quite. The way they'd distort spacetime would be different. A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass.

You also get differences in other things too, based on the other different physical properties and behaviours. Basically, if this exists, it's almost certainly a rogue planet or similar large body, not something exotic. It's not impossible that it's something else, but it'd be very very weird if that were the case.

>"A small black hole produces more concentrated distortion with nothing around it (think of a marble made of lead on a rubber sheet - lots of distortion, small object), whereas something large with the same mass (a bowling ball made of something very not-dense) would have a different shaped distortion, despite having the same mass."

I see what you are saying, but am thinking that the distortions need to coincide at a large distance from the object (ie at low accelerations), otherwise GR couldn't reduce to Newtonian mechanics. However, then I found this interesting discussion which leads me to believe that you in fact cannot really derive Newtonian mechanics from GR:

https://www.physicsforums.com/threads/advanced-books-papers-...

Where in the (observed) solar system is there space for a black hole, even a small one, considering the continued existence and observed orbits of the planets?

@stuff4ben: mission accomplished I guess? : )

Indeed. Thank-you petewailes! And I think I actually understood some of that too.

My pleasure :)

Thank you for the explanation, too!

Or you could google dark matter and find out why, don't treat HN like reddit, the snark wasn't needed. It's pretty straight forward this why his answer was correct.

I don't think it's obvious from a simple Google search why a material capable of explaining things about galactic orbits and large scale gravitational effects can't also explain localized orbital effects. There's no clear distinction in kind of effect, only scale.

Perhaps instead of not contributing (by shouting RTFM when there isn't a manual for beginns on that topic), you could give a basic summary of why that's the case.

If it's so simple, please outline in a line or two why dark matter can only impact large scale orbits (eg, galactic or bigger), and not cause localized or small scale effects (eg, solar system sized).