It appears obvious that if you add a 1D dimension that is perpendicular to all three observable dimensions and is a vector of the universe's expansion where most energy resides, then the universe expands at a constant rate in that 1D dimension and our observable 3D dimensions stretch by this vector in all directions simultaneously. However, we observe that the universe's expansion rate is accelerating, which suggests two things about our observations:
1. What we observe is under the time dilation effect, and each part of the universe we observe can have different rates of the time dilation effect due to the gravitational gradient of mass and the speed of matter (not the expansion speed). Thus, the universe is really expanding at a constant rate, and we are progressively slowpokes (our local time dilation effect is increasing... What could be the reason for this?).
2. There is no hidden dimension...
Adding an additional dimension of micron size transforms it into a 3D dimension. This is too far-stretched, even for a wild guesser like me...