I have found this:

https://msdn.microsoft.com/en-us/office/office365/howto/develop-office-graph

This technology sounds very promising and I understand that it is still in preview, but before I dive in too deeply, I was wondering if I can use this API (in the future) to search for experts (users) on a certain topic.

I have doubts that are caused by this line:

To present the most relevant content in different contexts, the Office graph uses a two-step analysis. First, it calculates which users in the graph are most relevant to the current context. Second, it retrieves the most relevant content associated with these users.
My Idea would imply the other way around ?
First find most relevant content and second find the user(s) who created it.

https://social.msdn.microsoft.com/Forums/office/en-US/2f2e9ed6-cd13-49f4-a838-f81d54456ed7/can-i-use-the-new-office-graph-api-to-search-for-experts-on-a-subject-?forum=appsforoffice