1. add cube

2. add UV Sphere

3. add geometry nodes to cube

4. scale down uv Sphere in edit mode

5. use this node tree

5. play a little bit with distance so that the spheres don't touch each other

result:

1. add cube

2. add UV Sphere

3. add geometry nodes to cube

4. scale down uv Sphere in edit mode

5. use this node tree

5. play a little bit with distance so that the spheres don't touch each other

result:

UPDATE

1. add a cube, delete top face and add solidify modifier, rigid body, passive

2. add a uvSphere, give it a rigid body, shape: Sphere, Sensitivity: collision margin: 0

3. copy that sphere as often as you need it and move it so it doesn't intersect with the others

4. run the animation until you are satisfied, then select all spheres and press object->apply-> visual transform

5. now you can delete all rigid bodies and do whatever you want to do with it

result:

video tutorial:

https://youtu.be/bQdIfseupK4