RE: Do we still need to build models beyond the Standard Model of particle physics ?

Note that this may be my last answer for the next 3-4 weeks as I will soon take some long offline time. I will come back to you at my return or during the break if I have enough strength to connect.

[…] I am unaware of any source of this creation of spacetime, of any cost of it, or of any disturbance it's birth may cause. There's no explanation for it at all, but it fits the observation that the universe is expanding in the way it is.

I don’t understand what you mean by “creation of spacetime”. The universe is in a state of accelerated expansion (a fact). What are the cosmic ingredients behind this? Well we don’t know and this is an open question.

How there can be a spacetime continuum that is complete, yet erupts from the vacuum in bits and pieces, I can neither explain. It seems that observation trumps speculation and things are what they are whether we can derive the reason they are that way or not.

What do you mean by “complete”? I don’t understand this word in this context. If you discuss the ingredients I mentioned above, then well, we don’t know.

The visible mass observed is calculated to be insufficient to account for observed motion of galactic arms, or the collision of the Bullet Cluster, and these are amongst the observations that are the basis for theories proposing dark matter.

Such observations depend on estimations of how matter is distributed across time and space. That estimation clearly is undertaken and considered rigorous enough to have mathematical validity, or such claims would not bother physicists much, as you state.

There is no fundamental principle behind the model, but it works. Moreover, the standard model of cosmology is mathematically sound, and you seem to ignore structure formation and the cosmic microwave background that are important indirect support for it. From there, finding the fundamental principles behind all of this consist of open questions. Many work on this.

In other words, the standard model offers us a way to get an idea about the true nature of cosmology and to make predictions in the meantime we have a better framework that does at least as good.

Just stating that ordinary matter satisfies those estimations isn't any different, except in that it doesn't require adding to the bestiary of particles in the standard model, and treats spacetime as Einstein showed it to be, a unitary field, a continuum.

Maybe. Then show me the associated predictions and let's compare them with data. As in the beginning of this thread, I don’t understand how the future could impact the present (even within a curved spacetime).

While the effect of gravity across time seems a novel concept, it is the only way gravity occurs. There is no universal instant of time across the spacetime continuum, but an objects time and movement through it is relative to it's movement through space. All gravity has to be effected across time, because time for every massive object is unique to it's inertial frame, and there is no universal instant of shared time. All time is relative. The inverse square law is sufficient to cover all gravity, since all gravity is effected across time.

Again, how could we integrate the future in calculations? Even if time is relative, causality and the arrow of time are important. This applies to the rest of the previous comment: we observe events today, that date from the past (light takes time to travel). I don’t see how to explain present observations with future ones.

I need to see the theoretical formulation to try to understand this. It is clear that gravity and spacetime are related and no one has ever said that we should ignore relativistic effects. This is not the problem. The problem is how something happening in the future could affect us today.