THREE.JS Skybox not displayed on screen

I am trying to include a skybox to a scene in a web page, so I followed a tutorial I found, everything seems to work correctly as my console (on firefox) doesn’t display any errors or warning.

First the code creates an array of texture, then assembles the textures in a box and finally adds the box to the scene. I don’t have any light in my scene as every objects are visible without any so far (and when I try to add one it doesn’t change anything, and the console doesn’t show any error either).

I thought it could be a matter of distance of the camera to the skybox so I got the skybox closer to the camera but still nothing.

I put my code down here if you want to see what I did so far. Thanks in advance for your help!

var camera, scene, rendu;
var r, t;

init();

function init() {
    r = 3;
    t = 1.1;
    // ---------- scene et camera --------- //
    camera = new THREE.PerspectiveCamera( 70 , window.innerWidth / window.innerHeight , 0.01 , 2000 );
    camera.position.set( 0 , 0 , 4 );
    camera.lookAt(new THREE.Vector3( 0, 0, 0 ));

    scene = new THREE.Scene();

    loadSkybox();

    // ---------- rendu ------------- //
    rendu = new THREE.WebGLRenderer( { antialias: true} );
    rendu.setSize( window.innerWidth, window.innerHeight );
    rendu.setPixelRatio(window.devicePixelRatio);
    rendu.setAnimationLoop( animation );
    document.body.appendChild( rendu.domElement );

}

function animation() {
    rendu.render( scene, camera );
}


function createPathStrings(filename) {
    const basePath = "./ulukai/";
    const baseFilename = basePath + filename;
    const fileType = ".png";
    const sides = ["ft", "bk", "up", "dn", "rt", "lf"];
    const pathStrings = sides.map(side => {
      return baseFilename + "_" + side + fileType;
    });

    return pathStrings;

}

function createMaterialArray(filename) {
  const skyboxImagepaths = createPathStrings(filename);
  const materialArray = skyboxImagepaths.map(image => {
    let texture = new THREE.TextureLoader().load(image);

    return new THREE.MeshBasicMaterial({ map: texture, side: THREE.DoubleSide });
  });

  return materialArray;

}

function loadSkybox() {
    // ----------- skybox -------------- //
    var skyboxImage = "corona";
    skyboxGeo = new THREE.BoxGeometry(1000, 1000, 1000);
    skybox = new THREE.Mesh(skyboxGeo, createMaterialArray(skyboxImage));
    scene.add(skybox);
}