Whitespace is a design element, not something that you need to try and fill.
When looking at a design, one client might say “oh, there is so much wasted space!” or some might say, “the colors are great but I think we need to put something to make it look lighter for the eye”. Both of them are referring to one thing:White Space.
Oftentimes, white spaces are overlooked.
The above quote comes from 11 Reasons Why White Spaces are Good in Graphic Design. I would also recommend reading Whitespace in Web Design: What It Is and Why You Should Use It
In short, you should use whitespace to help focus you users' attention on the purpose of your page.
Imagine what Google's homepage would have looked like had all the whitespace been filled. The whitespace adds to it, but removing what does not add to the purpose of the page.