Flexbox

align-items: center;

Resize the screen

CSS
.container {
  display: flex;
  height: 600px;
  flex-wrap: wrap;
  justify-content: center;
  align-items: center;
}
Box-1
Box-2
Box-3
Box-4
Box-5
Box-6
Box-7
Box-8
Box-9

align-content: center;

CSS
.container {
  display: flex;
  height: 600px;
  flex-wrap: wrap;
  justify-content: center;
  align-content: center;
}
Box-1
Box-2
Box-3
Box-4
Box-5
Box-6
Box-7
Box-8
Box-9