# Can dark matter accumulate at Lagrange points?

Interplanetary dust can accumulate at Lagrange points .

"Kordylewski cloud - Wikipedia" https://en.m.wikipedia.org/wiki/Kordylewski_cloud

But can dark matter accumulate at Lagrange points and is there evidence for this for example between pairs of galaxies or between the earth and moon?

If dark matter is dissipationless and only interacts gravitationally, then no it can't. How could it be brought to approximate rest$$^1$$ with respect to a Lagrangian point, its inertia would mean it just carried on with whatever kinetic energy it had?

If dark matter is weakly interacting then it might be possible for it to accumulate in deep gravitational wells formed by ultra-dense matter - i.e. in white dwarfs or neutron stars. But such ultra-dense matter does not exist at Lagrangian points in the Solar System.

$$^1$$ Something "at Lagrangian points" (the wording of the question) is necessarily at rest on average with respect to those Lagrangian points. They are actually effectively in orbit around the more massive body. There is no mechanism to decelerate and capture dark matter into an orbit with a speed of $$\sim 30$$ km/s (e.g. for an orbit near Earth) when dark matter is travelling past the Sun with typical speeds of hundreds of km/s.

Even should one argue that the dark matter has a distribution of speeds and some of it will be travelling at close to zero velocity with respect to the Sun, that matter will be accelerated towards the Sun as it approaches and would stil not be captured at the Lagrangian points.

• The smoother than expected profile of dark matter suggests to me that it does dissipate phys.org/news/2016-12-dark-smoother.html
– user51304
Commented Jun 21, 2023 at 12:07
• Huh? It's less clumpy than expected. @DaveTheWave Commented Jun 21, 2023 at 12:15
• temporary, reversible (-1) because I think you are oversimplifying the problem in order to write an easy answer. Associated with the regions of Lagrange points are unstable orbits that one can enter, then spend some substantial time in the general vicinity of a Lagrange point and the exit. That mechanism could result in a higher concentration without require inter-particle collisions or other lossy interactions. Accumulation ≠ trapped, much less "at rest" as you stipulate.
– uhoh
Commented Jun 21, 2023 at 21:57
• Accumulate means rate of particles entering a volume > rate of particles out that volume @uhoh Commented Jun 21, 2023 at 22:00
• No, that's what currently accumulating means. Having accumulated means there's an excess density, but it could certainly be steady state. And it does not require "trapped" or "at rest" Do all explanataions for a ""Kordylewski cloud" require dust-dust collisions for its formation? (I don't know, but it's a good question)
– uhoh
Commented Jun 21, 2023 at 22:05