I believe this phenomenon is an effect of teriifying things you have seen in the past. I believe you are still dreaming in this phase.
From my own dreams I know that something that happens while you are sleeping can be interwoven into the dream although you don'tactually see/hear/feel it. You can dream about a fire while it's actually burning around you. You can dream of an alarm bell while actually your clock tries to wake you up.
Beside that, many events in your life can leave a mark in your brain although you didn't actively see them. The brain stores more information than you know and it tries to sort it while you are sleeping.
But there is still the mystery that so many people see these dark hooded figures and they are always similar. The size varies but size is just the distance to your eye. If you have seen the real event with only one eye, your brain has no information about the actual distance of the object.
To go into the crazy conspiracy theories, there seem to be dark hooded persons all around the world, working on some secret goals. All of them look equally (or it is just one who can travel very fast all around the world) and they usually have a threatening appearance. Maybe all people who have these dreams had their memory erased in the past and that is the remains that couldn't be erased. But if so, the same organisation would also try to remove all discussions about this phenomenon.
Let's go back to reality. The descriptions often say the dark figures have no real face ("no face or defining features" like you said it). Hooded could simply mean the outline goes from the head straight to the shoulders without a visible neck. My idea is: It's a shadow, seen from a special angle in the corner of your eye. Shadows have no defining features, onlythe texture of the material below them. Because of the strange angle and corner of your view, the brain malfunctioned and interpreted it as a humanoid creature on its own, without brining the image to consciousness. Later, while dreaming (or wake-dreaming), your brain processes the remembrance of the "creature" it has seen.
Sorry for the long post but I found it interesting to think about the topic.
There are also cases that you can become temporarily paralyzed when waking up from remember sleep, which might have put you in a panic-mode which caused objects in your peroferal vision become exaggerated to look like humans, like sinthoniel said.