What it is
Cream soured with the same lactic cultures as yogurt — full-fat and cultured is real food; "light" is starch and gums doing an impression of it.
Why your biology objects
Lactic fermentation applied to cream, so you get live cultures plus the fat-soluble vitamins butterfat carries. Crème fraîche's higher fat is why it won't curdle in a hot sauce. The low-fat versions add modified starch and guar to fake the body the fat gave for free.