Is there any sin in a woman replying to scientific and cultural topics online that were written by men?

The default is that there should be no relationship between a foreign man and woman, except out of necessity, and to the extent required, and within the bounds of decorum. There is no objection to a woman responding to a man, or vice versa, in matters of knowledge, provided it adheres to the regulations of Islamic law. Indeed, the Companions - may Allah be pleased with them - both men and women, would respond to each other in matters of knowledge. Imam Malik said: "There is no one except that his saying can be taken from or rejected, except the occupant of this grave [meaning the Prophet Muhammad (peace be upon him)]."