#particles-js {
    /* stay in a constant spot on the page, regardless of scroll */
    position: fixed;
    /* fill the entire page */
    width: 100%;
    height: 100%;
    /* remove the space along the edges */ 
    margin: 0;
    padding: 0;
    /* position it to start at the top of the page */
    top: 0;
    /* put it behind other content */
    z-index: -1;
}