That is actually normal, and is not anything to worry about once you understand that SQL Server will use as much memory as you allow. See, serving data from memory is more efficient than serving from disk so SQL Server attempts to keep as much of your data in memory as possible.
An important thing for you to do right away is to set the "max server memory" server configuration to a sensible value that leaves enough memory for the Windows and other non-buffer pool SQL Server needs. Please read chapter 4 of this book (buy from Amazon or free eBook download) for details on memory management
There are no special teachers of virtue, because virtue is taught by the whole community.
--Plato