Every database in SQL Server which is getting read by the user or application is retrieved from the buffer cache. If the pages which are not in buffer cache are read by the user/application, those pages will be loaded into buffer cache first and then retrieved to the user/application.
Database Pages which occupied in the cache are retrieved by the following query:
SELECT count(*)*8/1024 AS 'Cached Size (MB)', CASE database_id WHEN 32767 THEN 'ResourceDb' ELSE db_name(database_id) END AS 'Database' FROM sys.dm_os_buffer_descriptors GROUP BY db_name(database_id) ,database_id ORDER BY 'Cached Size (MB)' DESC