Endosymbiotic theory holds that chloroplasts and mitochondria came about through the evolution of blue-green algae and bacteria through endocytosis. Endocytosis occurs when a substance passes into a cell through the cell's membrane, and then the cell plasma fuses together to keep the material inside forming an intracellular vesicle.
Endosymbiotic theory can be simplified for non-scientists and science students by saying that there were two prokaryotic cells; the one cell ate the other cell in order to form a double membrane. The evidence for this theory shows that it is highly possible that chloroplasts and mitochondria were primitive bacterial cells at one time.
The theory rests on the knowledge that both chloroplasts and mitochondria have similar attributes to other bacteria cells. They both have their own DNA that is separated from the nucleus DNA, and both also produce proteins and enzymes in order to function. To further support the theory, they each have a double membrane.
It is important to remember that a theory in the scientific field is more than a mere thought and is based around compelling evidence. It is considered to be an established explanation and is verified by the scientific community. Most theories are therefore considered as factual within the scientific community.