  In a strongly correlated system, collective excitations contain key
information regarding the electronic order of the underlying ground state. An
abundance of collective modes in the spin and valley isospin channels of
magic-angle graphene moir\'e bands has been alluded to by a series of recent
experiments. However, direct observation of collective excitations has remained
elusive due to the lack of a spin probe. In this work, we use a
resistively-detected electron spin resonance technique to look for low-energy
collective excitations in magic-angle twisted bilayer graphene. We report
direct observation of collective modes in the form of microwave-induced
resonance near half filling of the moir\'e flatbands. The frequency-magnetic
field dependence of these resonance modes sheds light onto the nature of
intervalley spin coupling, allowing us to extract parameters such as
intervalley exchange interaction and spin stiffness. Two independent
observations testify that the generation and detection of the microwave
resonance relies on the strong correlation within the flat moir\'e energy band.
First, the onset of robust resonance response coincides with the spontaneous
flavor polarization at half moir\'e filling, and remains absent in the density
range where the underlying Fermi surface is isospin unpolarized. Second, we
performed the same resonance measurement on graphene monolayer and bilayer
samples, including twisted bilayer with a large twist angle, where flatband
physics is absent. We observe no indication of resonance response in these
samples across a large range of carrier density, microwave frequency and power.
A natural explanation is that the resonance response near the magic angle
originates from "Dirac revivals" and the resulting isospin order.
