[CS:GO] Shadows in Hammer

I've just been messing around with Hammer, and after compiling and running my map there seems to be (what looks like) a black shadow around two of the walls for no particular reason. It fades the further away you get, so I tweaked the render distance settings to see if that was causing it but no change.

I checked to see if it was a leak but I don't have any errors when compiling, I'm most likely missing something completely obvious.

Any help for a beginner?

edit: Also tried different textures, no luck.

I've been following a few guides online and they haven't mentioned anything about lighting up to this point. Noob question: If it's lighting thats the problem, why do some of the textures inside the building show fine?

Here's the video I'm following: http://youtu.be/9BxKIodPme0?t=5m12s

So, to fix the problem I just need to add light entities?

Thanks guys, got it working after adding a light_env, tweaking the skybox settings and adding temporary light entities to the rooms. I wish mat_fullbright 1 worked for quick testing, but oh well!