I'm new to this impressive forum and have a question that may have been addressed a thousand times, but here goes. A FLRW metric is happy with a time-varying scale factor a(t) and zero curvature parameter k and could care less about density. The combination of a FLRW metric and the Einstein field equations, however, requires that a flat universe either not expand or that it have a density equal to a critical density much larger than independent observations support. The constraint is imposed by the Friedmann equation, k = Ho^2 (density ratio - 1). For a flat space, either Ho is zero or density is equal to critical. That strikes me as strange. Matter appears to flatten an expanding space rather than curve it. Does someone have an intuitive explanation of this phenomenon?