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"Bytram" writes:

Dwarf galaxies are causing a stir in the universe and are calling Dark Matter into question. Two stories on phys.org highlight the problem.

The first is: Map of universe questioned: Dwarf galaxies don't fit standard model and the second is: Did Andromeda crash into the Milky Way 10 billion years ago?.

It was thought that Dark Matter was required in order provide sufficient gravitational attraction to overcome the expansion of the cosmos. This is the Lambda-CDM model (i.e. the Standard Model of cosmology). One way to explain the discrepancy lies in MOND (Modified Newtonian Dynamics) which was developed by Mordehai Milgrom.

The Standard Model would predict satellite dwarf galaxies in the Milky Way and Andromeda galaxies to be widely distributed and would have to move in random directions. What the research is finding, instead, is that these satellite dwarf galaxies are located in a huge disk and are all moving in the same direction. This is much like what we see in our own solar system, but on a vastly larger scale.

The proposed solution to this discrepancy is that an earlier collision between the Milky Way and Andromeda galaxies ripped material from the galaxies and threw it a great distance. The observed dwarf galaxies are thought to have been formed in this debris.