  Mechanical strain is a powerful tuning knob for excitons, Coulomb-bound
electron-hole complexes dominating optical properties of two-dimensional
semiconductors. While the strain response of bright free excitons is broadly
understood, the behavior of dark free excitons (long-lived excitations that
generally do not couple to light due to spin and momentum conservation) or
localized excitons related to defects remains mostly unexplored. Here, we
develop a technique capable of straining pristine suspended WSe2 kept at
cryogenic temperatures up to 3\% to study the strain behavior of these fragile
many-body states. We find that under the application of strain, dark and
localized excitons in monolayer WSe2 - a prototypical 2D semiconductor - are
brought into energetic resonance, forming a new hybrid state that inherits the
properties of the constituent species. The characteristics of the hybridized
state, including an order-of-magnitude enhanced light/matter coupling,
avoided-crossing energy shifts, and strain tunability of many-body
interactions, are all supported by first-principles calculations. The
hybridized exciton reported here may play a critical role in the operation of
single quantum emitters based on WSe2. Furthermore, the techniques we developed
may be used to fingerprint unidentified excitonic states
