body, html {
    height: 100%;
    margin: 0;
}

.bg { 
  /* The image used */
  background-image: url(../img/background/galaxy.jpg);

  /* Full height */
  height: 100%;

  /* Center and scale the image nicely */
  background-position: center;
  background-repeat: no-repeat;
  background-size: cover;
}