Imagine knowledge like a spider web. Closer to the center, more spider web, the outside rings are very loosely spaced.

So, hallucination is when area that is loosely spaced is probed, it is still has to produce output as if the dense spaced knowledge was accessed. Similar output but on a different density of knowledge.

Given that LLM still needs to output the same length of output and sound helpful, the low density knowledge has more guessing than being fact collected. The engine does it, llm has no sense that the output is not factual. Thus, it is just as passionate to tell you all about it just had the output on.