Do conferences accept entry of those who didn't register before the deadline? A relatively big conference will take place in my neighborhood, but I haven't noticed of it until now. I'd like to attend some lectures done by young researchers who recently made a breakthrough. 

Unfortunately, the deadline of registration is over. Thus, I have this question: do conferences accept entry of those who didn't register before the deadline?
Most large scientific conferences will allow you to register at any time, even while the conference is running.  In this case the "deadline" is instead one or more deadlines for getting reduced rate registrations: they charge you more, the later that you register in order to get most to register early and have a good estimate of attendee numbers.

Some conferences, however, have size limits or require advance planning of other sorts, and so actually do have a strict deadline.  This is more often the case with small meetings than large ones, but the only way to know for certain for a particular conference is to check their website and, if it doesn't have information, contact the organizers.