What about astronomical observations makes scientists believe our universe is flat, at least as far as they can tell? Despite the critical Friedmann density being less than one, plus the existence of dark energy, etc.?
The spacetime geometry of the universe is determined by the total energy density, which includes contributions from matter (including dark matter), dark energy, radiation and curvature (if non-zero).
In the present-day universe the sum of these, dominated by matter and dark energy, adds up to something that is consistent with the critical density and the universe must be very nearly flat.
I think the particular observations that are highly constraining are detailed modelling of the cosmic microwave background angular fluctuations, combined with baryon acoustic oscillation data. These suggest that any curvature energy density is limited to 0.2% of the critical density (1 sigma) - Planck collaboration 2018.