Question: I just cut an object in half and then mirrored it. 
Before i joined the objects, they looked perfectly similar, but after joining, one half is darker than the other.
I tried some stuff and i did delete doubles, and i checked the material
but it all doesn't help

That's the thing, after joining the 2 halves:
[![thats the thing, after joining the 2 halves](https://i.stack.imgur.com/hDSH7.png)](https://i.stack.imgur.com/hDSH7.png)

Answer: 
I have 2 possible solutions. One, try (in edit mode) pressing {control n} which will recalculate the normals. If the normals are incorrect then it will fix that. Otherwise, it's just a question of shading.
Remove Mirror modifier.
Object Mode > CRTL+A,
Apply all transformation,
then add Mirror Modifier again.

Hahahahaha can't believe I remember that 4 seconds to some videos I watch.