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Hybrid light-matter states, polaritons, are one of the central concepts in modern quantum optics and condensed matter physics. Polaritons emerge as a result of strong interaction between an optical mode and a material resonance, which is frequently realized in molecular, van der Waals, or solid-state platforms (1-7). However, this route requires accurate (nano)fabrication and often lacks simple means for tunability, which could be disadvantageous in some applications. Here, we use a different approach to realize polaritonic states by employing a stable equilibrium between two parallel gold nanoflakes in an aqueous solution (8). Such plates form a self-assembled Fabry-Perot microcavity with the fundamental optical mode in the visible spectral range. The equilibrium distance between the plates is determined by a balance between attractive Casimir and repulsive electrostatic forces (9-11) and can be controlled by concentration of ligand molecules in the solution, temperature, and light pressure, which allows active and facile tuning of the cavity resonance by external stimuli. Using this Casimir approach, we demonstrate self-assembled polaritons by placing an excitonic medium in the microcavity region, as well as observe their laser-induced modulations in and out of the strong coupling regime. These Casimir microcavities can be used as sensitive and tunable polaritonic platforms for a variety of applications, including opto-mechanics (12), nanomachinery (13), and cavity-induced effects, like polaritonic chemistry (14).