It seems that you have a lot of confusion about the observable universe, dark matter and etc.
So, we can "see" 13.7 billion (intentionally not including expansion) light years in all directions and we "see" a redshift.
We can see further than that. The universe is 13.7 billion years old but due to the expansion of the universe, we can see further than the 13.7 Billion ly. The radius of the observable universe is actually 46.6 Billion ly.
What if, the reason we can't observe photons beyond this limit is that they decay into a low energy-high mass dark matter particle after 13.7 billion years?
We can't see beyond the observable universe because simply the light from there has not reached us yet. It has nothing to do with DM.
I have a hard time understanding/believing that the earth is at the center of our observable universe. (i.e 13.7 billion light years in all directions from us)
There's no center for the universe. We cannot define center in a homogeneous and isotropic universe. It's just a contradictory idea with homogeneity and isotropy. Look this The Cosmological Principle
For the observable umiverse we can define ourselfs as in the center due to the particle horizon.
Perhaps our universe is 20 times older than 13.7 billion years and the decay from the past light is the dark matter we seek, and our universe is not expanding, and we do not need to find dark energy to explain expansion.
Our universe cannot be 20 times older. The CMBR radiation, redshift data from other galaxies (Hubble Law), and the Friedmann Equation can give a highly good result about the age of the universe, And its 13.7 Billion year.
Photons do not decay.