Realistic game

I am making a first person shooting game

I already made the terrain and imported some objects

But my game is still not realistic and the buildings in the game are so dark

even though I adjusted the light

is there is any way that enable me to make my. game realistic?

you can do the lighting and shadows in your favorite 3D program and bake them into the objects. or you can buy Unity Pro and get real time shadows. also play around with the different materials. some materials work better than others, depending on what you are trying to achieve.