In addition to the NextGen gallery plugin, are you using any caching plugins? Also, have you done any recent upgrades/updates to your theme or WordPress install or plugins right before the problem started happening?
If you’re able to, try disabling all the plugins – especially if you have a caching plugin – and re-enabling all of them back again and see what happens.
From the looks of it, my hunch says that the issue is either one involving the integration of the NextGen plugin with the theme or one involving the CSS of some part of your theme, or a combination of both.
The reason I say this is because when I looked at your source code (and yes, I saw the blank space myself), I saw that the entire blank space is all related to the section where you have your NextGen galleries set up. Missing NextGen galleries point to an investigation of the NextGen plugin and any other plugins that might affect NextGen (like caching).
Validation for the page itself has some errors but they ought not do something like that, BUT the errors on CSS – especially since they’re cascading styles and with some of the errors creating parse errors – might affect such a thing, depending – especially if they are related to that missing block of content.
You have some missing semi colons and some of your comments broke the parsing and then in addition, there were several parse errors. If these CSS errors were related to the stylization of the blank space you’re seeing, it’s very possible that the CSS could be the culprit.
