What the gap is
The explanatory gap names a specific shortfall in our understanding. We can identify, in principle, exactly which brain events accompany a given experience: which cells fire when you feel pain, see red, or taste sugar. What we cannot seem to do is explain why those particular physical events are accompanied by those particular feelings, rather than by different feelings or by no feeling at all. The physical story appears complete on its own terms, yet the felt quality it is meant to explain still looks like an extra fact.
The philosopher Joseph Levine made the point with a now-classic example. Consider the statement that pain is the firing of C-fibers. Even if this identity is true, it does not let us understand why C-fiber firing should hurt. Contrast that with water is H2O: once you know the molecular structure, you can see why water boils, freezes, and dissolves salt as it does. The identity explains the properties. With pain, the corresponding sense of why remains missing.
Where it comes from
Levine introduced the phrase in a 1983 paper titled Materialism and Qualia: The Explanatory Gap. His aim was careful and limited. He did not claim to prove that consciousness is non-physical. He argued instead that even if physicalism is true, our explanations of experience fall short of the intelligibility we expect from good science.
Crucially, Levine framed the gap as epistemic rather than metaphysical: it is a gap in what we can understand and explain, not necessarily a gap in what exists. Mind and brain might well be one and the same, and the gap would still be there, because knowing the identity would not by itself make the connection feel explanatory. This distinction is why the explanatory gap is often discussed as the more cautious cousin of stronger anti-physicalist arguments.
Why it still matters
The explanatory gap is closely tied to what David Chalmers later called the hard problem of consciousness: the challenge of explaining why physical processing is accompanied by inner experience at all. Many writers treat closing the gap, that is, finding a genuinely satisfying account of why brains produce feelings, as the central unfinished task in the science of mind.
Opinions divide on what the gap signals. Some hold that it reflects a deep feature of reality that physical science, as currently conceived, cannot reach. Others argue that it is a temporary limit of human concepts or imagination, the kind of gap that earlier sciences also faced and eventually crossed as theories matured. A third camp suspects the gap is partly an illusion created by the peculiar way introspection presents our own states. The debate over whether the gap can be narrowed, dissolved, or must simply be accepted remains open.