Where is the configuration file? This is strange!

The documentation on running the Bitcoin daemon on a Ubuntu machine is clear. The configuration file is supposed to be at $HOME/.bitcoin/ location. And yet, I cannot locate it! I looked for clues at /etc, /var, and /tmp too; nothing there. In fact, in one of my attempts to launch the daemon, the debug.log (created at $HOME/.bitcoin/) did call out the configuration file to be $HOME/.bitcoin/bitcoin.conf but the file itself wouldn't be listed. Permissions? - I don't know.

Can you please tell me where to look for this file?

This is on Ubuntu-16.04, 64-bit where bitcoin was installed via PPA. Yes; I am working with Bitcoin code for the first time.
The configuration file is definitely in $HOME/.bitcoin/. If you cannot see it, the most likely explanation is that you have not created it. Just use your favorite text editor to do so. As far as I can tell, bitcoind -daemon will run without bitcoin.conf being present and you will still get the message 'Using config file /home/user/.bitcoin/bitcoin.conf' in debug.log even when no file exists.