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?