One half of my object is darker than the other 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:
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.