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Wording matters: why maybe physicists should reconsider using terms like 'probability' and 'uncertainty' when talking about quantum mechanics.
Elementary particles are neither waves nor (point-like) particles, nor are they both. The most general and unassuming description we have of their behaviour is formulated in terms of complementary states consisting of properties defined not in terms of specific values but as 'possibilities' (degrees of freedom). Interacting particles form a system wherein each particle's possible states are inseparably defined in such a way that the system as a whole "makes sense", and this is termed 'entanglement' between particles.
The "make sense" constraint is a restriction on quantum states to only allow those which preserve certain properties, for example:
- complementarity of states: if particles A and B are entangled and one of their properties are known to be opposites of each other, then no matter how, when or where you measure this property on any one of them, reality conspires deterministically in such a way that you will always observe the opposite result from performing the same measurement on the other,
- total energy: a composite particle that is split into subcomponents defines a system of entangled particles with all possible states giving the same total energy as the original particle,
- conservation of information: all knowledge one could ever have about an entangled system is embedded within that system in such a way that it could be theoretically retrieved at any given point along its evolution
We know, both from experiments and from our experience of the world (as a rather clearly defined reality with sharp boundaries and a seemingly linear history), that we are able to manipulate particle systems in ways that constrain their possible states to such degree that we can make definite statements about some of their properties.
How? Where did all those other possibilities go?
To answer this, one can invoke a variant of the anthropic principle known as the [anthropic bias](https://en.wikipedia.org/wiki/Anthropic_Bias_(book)). All coherent combinations of quantum states (i.e. all possible states of a system of entangled particles) define possible realities. The implications of this are controversial, but the statement can not be argued - all possibilities are clearly defined and equally valid - and each of those realities can have their own set of "observers", and each one of them could say "look, this particle is (approximately) here!". You and me could then very well be one of these.
Oops, forgot to add the decoherence part (actual answer to the question advertised).
As you find yourself always in one possible reality -- one fully coherent overall state defining you and all particles entangled with you -- it can be said that all other realities that evolved "in parallel" from a shared point in your history have 'decohered' from yours.
So the answer is: while we don't truly know, there is no reason to think those other realities "went" anywhere; they may be just as real as yours is and exist in parallel, overlapping to various degrees.
Some decohered realities merge back together with yours, and while it isn't proven to be 'the' explanation, you can still observe what the result of such a merging would look like: it would look like a particle behaving like a wave, as demonstrated in the double slit experiment.
#PBSSpacetime did a good video on decoherence, explaining this much better than I could: https://www.youtube.com/watch?v=GlOwJWJWPUs
