📌 background-size: cover

<body>
    <div class="box box1"></div>
    <div class="box box2"></div>
  </body>

.box1 {
        background-image: url("https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569__480.jpg");
        background-repeat: no-repeat;
        background-position: center;
        background-size: cover;
      }

.box2 {
        background: center/cover no-repeat
          url("https://cdn.pixabay.com/photo/2017/08/30/01/05/milky-way-2695569__480.jpg");
      }

한번에 축약형으로 작성도 가능하다.

동일한 결과