Well it's a bit complicated. You were in fact dropped here by a vortex, replacing someone else. If you go to this [website], it has all the information. People are here from various points in time and other universes. They also tend to take people away too.
A perfect example... I have kids. Four of them are here. I did have seven here, but three of them disappeared with their mother. There was another me here. Well male me. There is a female version of myself here.
Some of the things that happen here are... weird. Random singing, clothes melting off, turning into kids, turning into the opposite sex.
Oh and if you look in your pockets, you should have some ID. We replace people here so your ID will have your address on it of who you took the place of. Where are you?
[Martha] Just to give you a heads up... the Doctor is here. So's Rose. They recently got married.