The chloroplasts are the organelles in plant cells that provide energy to the cell. the equivalent organelle in eukaryotic (animal) cells is the mitochondria. both are also unusual in that they have double membranes and contain their own set of dna. what is the current explanation for this observation?

The current explanation for this is an endosymbiotic theory. It is believed that both, chloroplast and mitochondria were once prokaryotic cells (bacteria), living inside larger host cells. The host cells and bacteria formed endosymbiotic relationships long ago. Bacteria inside the host received protection. The aerobic bacteria (oxygen-using)  helped the host to exploit O2 for extracting energy and through millions of years became mitochondria. The photosynthetic bacteria have provided crucial nutrients for the host and later became chloroplasts.