Q:

Is there filler manga? 

When watching anime I often come across filler episodes or arcs.  They are often episodes that are not described in the manga. 
I haven't read much manga, so I was wondering: Does manga also have filler volumes? 

A:

Depending on your definition of filler, I would say yes, there are indeed filler arcs in manga. 
I say this for a few reasons. 

The other answer on here as I write this assumes that the manga is always the original content, which is untrue.  While that's the most common way to do things (adapt manga into anime) there are certainly cases of adapting anime into manga form (for example, Madoka Magica). 
Consider this TV Tropes page.  It offers a few definitions of filler.  One is something that "isn't in the source material."  However it could also be considered something "unrelated to the main plot," which doesn't necessarily mean that filler arcs are exclusive a specific adaptation.  

For example, consider the Excalibur arcs in the Soul Eater manga, which also appear (almost verbatim in some cases) in the anime.  They have no practical effect on the overall story - they exist simply for comedic effect.  If you skipped those chapters, the overall story would not really lose anything. 
Therefore, yes, manga can have filler, because filler can be defined as anything which does not have a direct effect on the main story.