It's hard to tell for sure at that resolution, but it looks like overly aggressive sharpening (unsharp masking) to me -- you are probably taking JPEGs straight from the camera, right?
Unsharp masking works by "forcing" the contrast at the edges -- it makes the darks slightly darker and the lights slightly lighter where they meet in order to increase the difference between them and make the edges more noticeable.
You can either shot in RAW format and take control of sharpening in post-processing or (and this should be possible with the 550D/T2i) go to the settings on the camera and reduce the amount of sharpening applied automatically to JPEG images (if you want to continue using JPEGs). I'm not familiar with that model's menu system, but the manual should tell you where you want to go.