Aerosols are common in the atmospheres of exoplanets across a wide swath of temperatures, masses and ages1-3. These aerosols strongly impact observations of transmitted, reflected and emitted light from exoplanets, obfuscating our understanding of exoplanet thermal structure and composition4-6. Knowing the dominant aerosol composition would facilitate interpretations of exoplanet observations and theoretical understanding of their atmospheres. A variety of compositions have been proposed, including metal oxides and sulfides, iron, chromium, sulfur and hydrocarbons7-11. However, the relative contributions of these species to exoplanet aerosol opacity is unknown. Here we show that the aerosol composition of giant exoplanets observed in transmission is dominated by silicates and hydrocarbons. By constraining an aerosol microphysics model with trends in giant exoplanet transmission spectra, we find that silicates dominate aerosol opacity above planetary equilibrium temperatures of 950 K due to low nucleation energy barriers and high elemental abundances, while hydrocarbon aerosols dominate below 950 K due to an increase in methane abundance. Our results are robust to variations in planet gravity and atmospheric metallicity within the range of most giant transiting exoplanets. We predict that spectral signatures of condensed silicates in the mid-infrared are most prominent for hot (>1,600 K), low-gravity (<10 m s-2) objects.