SQL Server will take every single gb of memory that you assign it. It won't release that memory unless forced. It's very normal for servers to run with high memory use. You have to worry about other metrics around memory to understand if you're hitting memory issues, not just the amount in use.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning