Astronomers noticed odd ripples of dust in the Milky Way in 2009, making many feel a disruptive force was present. However, they could not determine at the time what caused the issue. By 2015, we had an answer. The cause was a dwarf galaxy orbiting the Milky Way, which is ever so slightly altering the motion of our galaxy with its gravitational pull. Scientists were only able to see it due to 4 bright stars around the galaxy.
It would be too hard to see without it. Astronomers believe dark matter is the reason why we cannot truly see it very well. Given the name Galaxy X, the dwarf galaxy was unique at the time. This all changed when in 2016, we discovered Dragonfly 44, made of literally 99.99% dark matter. Along with Segue 1, the dwarfs have been spotted more and more. However, no one knows how they came to be. Moreover, what their role is with dark matter.