Negative Space.

"Negative space is, quite simply, the space that surrounds an object in a image. Just as important as that object itself, negative space helps to define the boundaries of positive space and brings balance to a composition."

https://www.creativebloq.com/art/art-negative-space-8133765

Negative space is the space that is not filled by matter, however can be used to define and accentuate the positive space. However, it can also be used to create shapes and illusions which can give subliminal messages within a picture. If this is done effectively it can result in a more visually engaging image, with the hidden meaning and layers behind the design.

In my opinion, this Pittsburgh Zoo logo is an excellent portrayal of the creative use of negative space, because it entails a lot of detail to visually describe the zoo, while still giving minimalistic vibes - this keeps the mind occupied without overcrowding the image.