They are described more as Outside /all/ reality, outside the whole multiverse. Rather than a parallel dimension of the multiverse, "outsiders are the outsiders and they just sort of look different depending on which universe they are trying to get into and destroy." (WOJ, 2015)  They were there before Creation got going, and are trying to return things to that nice clean primordial state.   JB described them as the "generic Hellboy fangs and tentacles crowd" (WOJ).  Also, just like Bram Stoker on the Black Court Vampires, HP Lovecraft's work are considered real works based on actual secret knowledge in the setting.

Right, thanks that is a much better description of what I was trying to say.  Essentially, there are many, many monsters, evil beings, and dark gods of various mythologies that exist "inside" the Dresdenverse.  Separate from all those incredibly powerful evil or amoral beings are the Outsiders, who have their own desires to essentially destroy the universe.

My point in all that is that it doesn't really make sense that people could cross over back and forth between the "outside" and "inside".  A lot of people in this thread are suggesting that a human (or other being of the Dresdenverse) could somehow move beyond the Outer Gates or even go back and forth at will.  That doesn't really jive with what we know of the Outsiders.  If you could go back and forth that easily, it would have already been done en masse and the world would have been destroyed by the flood of Outsiders.  Maybe there is a plot twist that what we think are Outsiders influencing the real world are actually just a group of "Insiders" who have some kind of evil cult that want to open the gates and have no contact with or influence from the true Outsiders, but I think that would be kind of a lame plot twist personally.