How to remove those lines around the div?

Question

css

They may not be visible in the photo, but they are there. How to stretch a div to the whole page so that there are no these lines on the side and bottom?
CSS:

width:100%;
        background-color: #444849;
        min-height: 320px;

Answer 1

Thanks everyone! I figured it out, it was in the user agent stylesheet

Answer 2

body {
  margin: 0;
  padding: 0;
}