" 'I'm sorry, Doc!' Marty blurted. 'It's all my fault you're stuck back there!' The teenager's hands balled into fists as he remembered whatever it was that had happened. 'If only I hadn't been late!' / Doc shook his head, half to let Marty know things weren't as bad as they seemed, half to ward off the teenager telling him anything else about the future of Doctor Emmett L. Brown. / 'Well,' he said reassuringly, 'there are plenty worse places to be than the Old West. What if I'd ended up in the Dark Ages?' He laughed ruefully. 'They would probably have burned me at the stake as a heretic or a warlock or something.' "
—From Back to the Future Part III by Craig Shaw Gardner (quote, pages 14 and 15)

The Middle Ages was a period of history which lasted from the 5th to the 15th century.

Marty McFly likened 1955 to the Middle Ages during his first visit to Lou's Cafe, after Lou Carruthers said he didn't have any artificial sweeteners like Sweet 'n' Low.[1]

In 1955, Emmett Brown, after hearing about his future self having crash-landed in 1885 after the DeLorean time machine was struck by lightning, was glad that he had not landed in the Dark Ages, as he thought the inhabitants might have burned him at the stake as "a heretic or a warlock or something".

On March 3, 1986, Marty, Jennifer and an amnesiac Doc visited the yard of Lee Bros. Towing & Repair, looking for the steam time car which Doc had arrived in. When they found the vehicle, Jennifer, unfamiliar with the 1890s design, stated it looked like something from the Middle Ages.