So endometriosis is just uterine lining growing outside the uterus?

Sort of, but endometriotic growths aren’t quite the same as the endometrial lining in the uterus. They’re not actually true endometrial tissue; they usually have differences that are visible under the microscope. Theories about development include “retrograde menstruation,” in which menses flow back up through the fallopian tubes and into the pelvic cavity; however, most women experience retrograde menstruation, and don’t experience endometriosis, and some people who have never menstruated have been diagnosed with endometriosis.^[1][2][3] Other theories involve transformation of other cells into endometriotic cells.^[4]