extend the white space in the theme TwentyEleven

start by creating a child theme of Twenty Eleven – https://codex.www.ads-software.com/Child_Themes

do not edit the default theme Twenty Eleven directly, as it is needed as a fallback in case of problems.

extend the white space that you can see in the page’s centre to extend for the whole page or some specified size

try to add some content (?)

PS:
it seems you might have validated the nesting of html comments, as your site link shows a virtually empty site;

https://validator.w3.org/check?uri=http%3A%2F%2Fcloudchowk.com%2Ffuture&charset=%28detect+automatically%29&doctype=Inline&group=0

https://htmlhelp.com/reference/wilbur/misc/comment.html

Thanks. That greatly helped me and i’m working on it.
Now i have created a basic child theme with a simple stylesheet as guided here
But it seems to have 6 errors in default when validated. I’m guessing these errors are those of the theme, since i did not edit any file of the parent theme except over-writing its style.css with that of a child theme.

Thanks in advance.

yes – those 6 errors are very unlikely to cause any problems.