Dimmer switch not connected to ground
The bare copper wires are the ground (grounds are either green or bare wires). You need to add the green wire from the dimmer to this bundle in the wirenut. 

The white wires are probably the neutral, providing a return from the lamp to your electrical panel. If you had a smart switch that needed a neutral connection, you would connect there.

Note: this comments assume you are in North America. Different countries use different insulation color conventions. Thanks, @Clonkex, for the reminder.