There is growing evidence that memories are actually stored in the brain in a very fragmented fashion, with individual fragments held in the place where the data point was processed. That is to say, your memory about driving to work this morning might consist of the sound of traffic and horns stored in the sound processing part of the brain, the route is stored in the part of the brain that processes spacial relationships, individuals signs stored in the parts that deal with vision, color, shape and contextual meaning, etc. These fragments are stored as archetypes: you do not have hundreds of memories about how bacon tastes, for example, you have only one or two that get used over and over again.
In the cerebellum, there is a kind of daemon that assembles these fragments into a cogent whole. This daemon is basically an idiot savant, capable of amazing feats but about as bright as a puppy. Like a puppy, it is very eager to please: if you ask for a memory that it does not know, it will assemble one for you out of the stored memory archetypes. There are independent checks, such as the part of the brain that gives rise to the “I’ve seen/heard/been here before” feeling, but ultimately the memory daemon is the final arbiter of what we remember.
If the daemon can be convinced that a memory exists, it will exist. Maybe it is an actual event — a birthday party or observation of a crime — with some facts remembered and other filled in to justify opinions or cover gaps. Maybe it never existed and was created from scratch, like a Ferris wheel made out of Tinker Toys. Once it has been sufficiently reinforced, it will be as real as any other memory.