This, like most of my images from the Sphere series, has its origin in a 2-dimensional picture. Below you see (parts of) the Apollonian Gasket. Descartes and Princess Elisabeth of the Palatinate discussed formulas for a fourth circle tangent to four other circles, more than 300 years circle packings became fashionable.
Now, in three dimensions, begin by placing four equally sized spheres into a larger sphere, like so:
Start filling the empty space with more spheres, each as large as possible to touch four other spheres.
and going, and going.
Finally, when you are tired and done, remove the first four largest spheres to create an empty space, and have a peek inside. What you see might look like the image at the top.