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[–] ShinyVoater 1 point 0 points (+1|-1) ago 

You don't have to see something make a complete orbit to know its period. For example, the distance from the center of a perfectly circular orbit can be determined by its distance from the center; increasing an object's speed simply serves to send it into a higher orbit with a longer period.

Galaxies are fairly well-rounded and should, by simple extrapolation, play by the same rules. Instead, the disc moves at a far more unified pace(like a CD), which requires mass to be where it visibly isn't. Some form of invisible matter is the only thing that makes sense with our current understanding of the universe. Alternate theories exist, of course, but, as I've said, they all have one problem or another that brings them out of agreement with observational evidence.

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[–] B3bomber ago 

If it takes 26,000 years to make 1 orbit, how are you going to tell me its 100% circular after seeing it for less than 100 years?