Is endosymbiosis a valid scientific theory? how?

The endosymbiotic theory is the accepted mechanism for how eukaryotic cells evolved from prokaryotic cells. First published by Lynn Margulis in the late 1960s, the Endosymbiont. Theory proposed that the main organelles of the eukaryotic cell were actually primitive prokaryotic cells that had been engulfed by a different, bigger prokaryotic cell.