Dark spot after replacing screen?

So i replaced the screen on my 4S (first repair on an iPhone i have done) and its gone okay, everything works just i couldn't quite get the front and back to fit snug and it took some effort to get the back on. The new screen works perfectly fine just there's a small dark spot in the middle right area of the display and i was wondering what might have caused it? (it doesn't really bother me the previous owner had badly scratched the original glass anyway so its still a vast improvement)

there were small gap when a fitted the new screen and it was a cheap £10 screen from ebay so it might just be like that or have a damaged it when fitting?

thanks in advance for your help :)