Why there is huge white space after in the header after adding theme option function

I’m writing a wordpress theme for some people to use it. Everything is doing fine until I add a theme options page to the admin control panel in hope that the user can easily manage their website display.

There’s a huge white space occurred in the header. You cannot see any html element with the right click and inspect element.

I know there’s table and form being add to the admin control panel. But why it’s going in there in the header of the theme? How to get rid of it?

Thanks,