It's based on large amounts of data and attaches a weighting to chances created and goals scored.How can you predict anything that's going to happen right down to numbers of goals and assists? seems nonsense to me
It is data, it's not perfect, but it's the best thing we have in football to determine how good a team or player is/has been, for example:
Against Wolverhampton we won 3-0, the data said after the match the expected goals was:
Man City 3.12 to Wolves 0.10 That's shockingly accurate, it's not always this good but over a long period of time (or minutes) it is quite accurate, here's another:
In the premier league we have scored 59 goals, the expected goals were 54.87, so 93% accurate
We have conceded 17 goals, the expected goals were 17.40, so 98-99% accurate