When I type this \"http://example.com/Hello%20There/\" , it displays the index page wich is : \"http://example.com/Hello%20There/index.html\" .
Well, what I want to
You can make a character optional by appending the ? quantifier to it like this:
RewriteRule ^([^/]+)/?$ $1/index.html
Now both /foobar and /foobar/ would be rewritten to /foobar/index.html.
But it would be better if you use just one spelling, with or without the trailing slash, and redirect the other one:
# remove trailing slash
RewriteRule (.+)/$ /$1 [L,R=301]
# add trailing slash
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .*[^/]$ /$1/ [L,R=301]
These rules either remove or add a missing trailing slash and do a permanent redirect.