Is it possible to make merit for dead we do not know? For all those who have recently died?
Usually merits are dedicated for one's ancestors seven generations back, good householder. Relatives are divided most far apart, and therefore good to invite all to rejoice with ones good deeds and also take part on the given.
As there are act-ually less occasion and realms where others could receive gifts - one of them is having given in the past - it's good to also better ones own conditions as one might not know where later one would arise.
Some detail explainings on where others might be able to benefit from one's gifts, the Sublime Buddha once shared in detail:
Then Janussonin the brahman went to the Blessed One and, on arrival, exchanged courteous greetings with him. After an exchange of friendly greetings & courtesies, he sat to one side. As he was sitting there, he said to the Blessed One, “Master Gotama, you know that we brahmans give gifts, make offerings, [saying,] 'May this gift accrue to our dead relatives. May our dead relatives partake of this gift.' Now, Master Gotama, does that gift accrue to our dead relatives? Do our dead relatives partake of that gift?”
“In possible places, brahman, it accrues to them, but not in impossible places.”
“And which, Master Gotama, are the possible places? Which are the impossible places?”...
May nobody ever miss to share ones good deeds, may nobody miss possibilities of generosity and deeds of gratitude and goodness.