Why Is Mac Malware Less Frequent Than On Windows Systems?
To answer your main question: malware is less common on Macs than Windows systems simply because people haven't written as much malware for Macs.

Any debates about which operating system is "more secure" than another one are not very meaningful. Neither OS X or Windows will ever be 100% free from security flaws, so there will always be potential vectors for malware -- not to mention trojans and social engineering. You need only look at the system updates to see that Apple is constantly patching security flaws in Mac OS X, just like everyone else.

If you were a malware author and wanted to get your malware onto as many systems as possible, wouldn't you target the OS with the largest user base? This is most likely why malware is much more common on Windows; so many more people have scrutinized it, looking for exploits.


  Is it true that not one single virus has been identified on Macs?


Assuming you mean "malware" in general, no, that's not true. In recent memory, the Flashback malware has been delivered by both a trojan installer (which tricks the user into giving it permission) and an exploit in Java (which doesn't require any user action).


  Is it also true that the majority of malware identified is in the form of trojans


I can't find any reliable data, but there have definitely been several trojans targeting Mac users in recent years.


  Is the underlying reason that malware is supposedly less is because Windows still dominates in usage terms for most people, but eventually in a few more years, there will be just as much malware on a Mac as on a Windows PC ?


It's hard to say for sure, obviously, because nobody can predict the future. Apple is obviously taking steps to try to keep Mac users as safe as possible (e.g., Gatekeeper), but again, OS X will never be 100% immune to security threats. It's just a fact.