Add a comma after decade because the introductory phrase is longer than a word or two. (You can use a comma with a shorter introductory phrase, too, but you don’t need to.) Add another comma after there and one more after them.
I changed passively observing the people to passive observation of the people because it shifts the meaning just enough that the first part of the sentence now fits better with the subject.
For the past decade, an intangible parallel universe overlapping our own has been slowly wavering into visibility. After a few years of passive observation of the people there, it becomes obvious that now we are becoming visible to them, as well.
(Sorry I don’t have the grammar jargon right now to explain this stuff in greater detail. I just finished editing a rather long-ish short story, and my brain is full of acronyms and sci-fi jargon instead… WordPress’ spelling and grammar checker is being stupid again, by the way. It wants me to change now fits better to no fits better. Yeah, ’cause that’s sooooo much more grammatically correct. *rolls eyes*)