Essentially. Minor spoilers ahoy matey.
So at the end of Dead Space 2 you basically have to stop the man made marker in the sprawl from starting convergence. Through out the first two games it was never really explicitly stated what this meant. Same with why everyone affected by the marker kept spouting "Make us whole." Anyway at the end you start to see a mad rush of necros going for the marker. This is the start.
The markers work by altering the genetic makeup of anyone in proximity, thus giving us necros. Then the necros spread, splashing death and blood along the way to allow for more necros and eventually a kind of critical mass is reached when enough people have been turned. This is the start of convergence. The end result is the brethren moons. You get a glipse of it when Issac completes the codex and he sees all the aliens from Tau Volantis being sucked into the sky and the moon starting to form. The science is iffy here, because they never really explain that there is some type of magnetic force that causes all the necros to converge on a location, presumably a marker (again presumably a black one, but not necessary as evidenced by every red marker...ever) but the attraction is implied in the cutscene.
If you were to apply some logic to the idea, you'd probably land somewhere along the lines of how if you have a large grouping of individuals, as in an entire planets population, that were 'magnetically' attracted to each other, the sheer force of all that mass would cause the matter to basically smash together. The stretch of it being that instead of a blob of blood and bone, the marker somehow reorganizes the matter into a single sentient being, the brethren moon. But I mean, you're playing a video game that basically has a rock turning people into shrieking un-dead things - its a matter of willful suspension of disbelief.
