Today, I noticed that the rear axel (the silver part, is that what it is called?) is wider than the rear dropout.

As a result, my skewer is not flat on the bike, leading the wheel to wobble a little.

How do I fix this? Would it be appropriate to put a few washers between the dropout and the hub?

The bike is a '10 CAAD9 and the wheel is a '12 Ksyrium Elite.

Thanks.