Searching for files in Dash brings up all the files I have used but none of those I haven't used yet. Shouldn't it show all files with the title I search for instead of just the ones I have already used?
The Files dash of Unity shows up the most recently used files as well as the favourite folders on the home page. This behavior is by design.
If you right-click the Files dash icon in the launcher, then you can select a particular type of file (such as Documents, Images, Presentations, etc) and the Dash will show up all of the recently used files of that type in your home folder.
There is a found lens that do deeper searches.