Yes thats a valid file but its been my experience that ... files markers that gits who have used root kits (or some exploit you haven't patched) have been in your system.

So I'd avoid using it. The reason why that its a valid file is to do with the way inodes work on Unix. The '.' 'points' to the local directory and '..' to the parent directory. This works in a chain right up to '/'. Consider that as everything in Unix is essentially a file and that only those two files are considered special then what you have created is a dot file called '..'.

Of course because its a dot file you have to use 'ls -la' to normally see them. Hence why they are left about in user directories. Also a reason to avoid them.

Now if you didn't create that file - I'd be worried :P.