We observe stars rotating in galaxies faster than they should for the mass we see. We observe spiral galactic arms moving very slowly as new stars get in the arms and old stars shoot forward. We assume Dark Matter, the invisible extra mass, to account for the rotation, but it is several times more than the total amount of stars and planets we see, and also of the mass we hypothesize but haven't seen yet. We still don't have a consensus about what we assume are the reasons why the galactic arms move so much slower than the stars with which they are made from.
What if we are wrong? What if Dark Matter is not the Sterile Neutrino or aliens hiding their stars? What if it is a simple issue of geometry, and its effect accumulating over time?
Proposing: Lightspeed Delay model of Dark Matter.
The idea in short: For objects several hundred lightyears away, gravity - spreading at lightspeed as per LIGO experimental verification - will inevitably exhibit lightspeed delay effects.
Simplest way to imagine it is how we see a plane flying in the horizon, but hear it in a different direction.
Like that, streams of stars traveling in parallel (from a bird's eye perspective) will see each other's ghosts of the past. The lightspeed delay makes it so that they drag and slow each other less (but cumulatively) than if they were pulling at each other instantaneously. We do not observe these effects within the solar system or practical experimentation because of proximity. The Sun being away from Jupiter only 30 minutes of light vs. several years of light for the closest star.
Each individual star pulls on each other's past ghost image, but a stream of stars, a galactic arm, or the rest of the galaxy, exhibit statistical behaviour also, on top of that, for example "filling in" for a star that moved on. I believe this is what makes for the manifestation of faster traveling stars and slower traveling galactic arms. - Which may imply that Dark Matter need not exist.
While the solution to the Dark Mater problem may be important, I wish you to also see 'Building A Universe Competition' #BAUniC baunic.blogspot.com and all over minds.com, open minded research towards the formula of the universe, much more important in my opinion.