Refer to the passage below and then answer the question afterwards in the same language as the passage:

Passage: Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories.  The characteristics of this fifth force depend on the theory being advanced. Many postulate a force roughly the strength of gravity (i.e. it is much weaker than electromagnetism or the nuclear forces) with a range of anywhere from less than a millimeter to cosmological scales. Another proposal is a new weak force, mediated by W′ and Z′ bosons.

Question: What natural phenomenon is the fifth force likened to?
gravity