Board Thread:General Discussion/@comment-348642-20160208222352/@comment-26123021-20160211110502

Monsters can create light sources with magic- the fire in Toriel's living room is very very heavily implied to be her fire magic. Towns like Snowdin and New Home probably have lamps and stuff, Waterfall is mostly dark, Hotland has lava, and the CORE probably glows from flashing lights and other mechanical stuff.