While your first reaction might be to flip out, it’s best to take a practical approach to the situation.
Are the two of you in the exact same job with matching job responsibilities? Do you have the same education level? Perhaps your coworker has a higher degree, extra certification, or more years of experience. These things matter and even though you might not want to hear it, could be the (very good) reason you're getting paid less.
After all, you can't complain to your boss that your colleague deserves a demotion and a reduced salary.