Sorry for the delayed answer, once again. Busy week, buy week… and this discussion is interesting. I cannot answer it in one line :)
Dark matter as I understand it's description is proposed to comprise of WIMPs, weakly interacting massive particles, which have properties specifically that do not otherwise interact with other matter than gravitationally.
WIMPs are only one possibility amongst many. Those are dark matter candidates particularly targeted at current experiments. But you have many many other options, ranging from very light particles to much heavier black holes. The only common ground is that they have gravitational effects. The rest belongs to the details of each model.
[…] ordinary matter at other times is simply not observable because we perceive time as an instant, and have not understood the nature of spacetime with which ordinary matter interacts.
We can observe it through the byproducts of what this matter did in the past. For instance: cosmic rays. We just get them on Earth after a while. For that reason, when we observe the universe far far away, we in fact observe the universe as it was a while in the past.
From the rest of what you mentioned, there is actually one point on which I disagree. The arrow of time cannot be inverted. We can observe the present and the past (which corresponds to observing far away), but not the future. If I have well understood, it seems that you want to consider what is happening now, in the past and in the future. This is not possible. We can account for the first two in the calculations (including what is happening on the way and during history), but not for the latter. How would an event happening in the future impact us? This is what needs to be clarified, and this may be in contradictions with the known laws of physics.