Slow moving background particles

body {
  background: #1E1E24;
}

.particle {
  width: 1px;
  height: 1px;
  background: #ffffff;
  position: absolute;
  top: 50%;
  left: 50%;
}

.layer {
  position: absolute;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
}

.layer:nth-child(1) {
  z-index: 1;
}

.layer:nth-child(2) {
  z-index: 2;
}

.layer:nth-child(3) {
  z-index: 3;
}

.layer:nth-child(4) {
  z-index: 4;
}

.layer:nth-child(5) {
  z-index: 5;
}

.layer:nth-child(6) {
  z-index: 6;
}

.layer:nth-child