Jesus definitely was not born on Dec. 25 (or on Jan 7, which is the Orthodox Christmas) No one knows the date of Jesus’ birth. Anyone who says they do is wrong, and should be ignored, in my opinion. The Bible gives us virtually no help in trying to discover the date. I assume that God did this on purpose. If it was important to know the date of his death, he would have given us the information. Perhaps it is because God does not want us to celebrate the date of the birth of Jesus. I think he would prefer that we celebrate his death and his resurrection. We know the dates of these events.
What the Bible does do is it tells us in the Gospel of Luke that when Jesus was born the shepherds were out in their fields with their flocks. Scholars tell us that this almost certainly was not in the winter, when shepherds stayed back in the towns and took care of the sheep in their pens. Scholars rule out the winter, and also tell us that the summer is less likely than the Fall or the Spring for the description of Luke. So, Jesus was most likely born in the Spring or the Fall of either 6 or 5 BC. We do not know the exact year, but we know that Jesus was born before Herod died in 4 BC, and given the description of Herod killing the male children near Bethlehem, the most likely year is not 4 BC but either 5 or 6 BC.
December 25 was chosen in Rome and therefore is the “Western” Christmas because it worked as a replacement of the Roman pagan feast of Saturnalia. The early Christians who proposed this date did not do so because they thought it was the actual birthday of the Savior, but as a counter-celebration to a pagan Roman holiday to help Christians avoid pagan idol worship. I do not know how the Eastern church settled on Jan. 7, but I can say for sure it is not because the Greek church actually believed that this is the actual birthday of Jesus of Nazareth.
God created antimatter for the same reason he created matter. Every aspect of the creation event we know of as the big bang was precisely ordered so that we could exist and that we could know God. Antimatter is as essential to the physical makeup of the universe as matter is. It is part of the balance in the universe. As the negative and the positive charges in the universe are perfectly in balance, so the matter and the antimatter are almost exactly balanced. The slight imbalance of the two leads to the actual physical matter that we observe. The excess of matter over antimatter creates the actual matter we experience. All this is part of the intricacy and the wisdom of the means by which God created the universe. It is a bit difficult to explain to a person who knows little about physics and all the different kinds of particles (electrons, quarks, muons, positrons, photons, neutrinos, etc.) why each one is needed for the universe to exist as a physical place that supports life, so to some extent you will probably have to take my word for it as a physicist that antimatter is as essential to life as matter.