An important part of good thinking is to understand the point at which something requires no further explanation. Most people are aware of the series of questions that a child may ask as explanations of the world are given to him or her. The child continually asks "Why?" no matter what answer is given. Usually the parent runs out of answers at some point and says something like "It just is!" or "Because I say so!". These are not good answers. They are, in the first case, unreasonable and in the second case a bold-faced lie. It is much more accurate and reasonable to say "I don't know" once you reach the point at which you have no more answers.
We should always expect that answers exist to the question 'Why?' (or 'How?') - it is an essential part of our reasoning nature. If at any stage in the explanation I answered by saying 'It just is.' I would rightly be accused of being unreasonable. No one could reason with me because I would be stating that there is no reason for the explanation I have just given. To take such a position is to assert that your explanation is the truth. This, besides being unreasonable, is very arrogant. It doesn't recognise the limitations of human knowledge as mentioned in the section on the basis of knowledge. In Islam the first characteristic of the faithful (muttaqin) is belief in the unseen (al-ghaib). This stresses that the first characteristic of a Muslim is acknowledgement that his knowledge is in principle limited and that part of reality is always unknown because it is unseen.
Asking 'why?' can be split into 2 meanings. The first is to mean 'How?'. This question digs ever deeper into understanding the causes and descriptions of reality. The second meaning is 'So What?!' and boils down to asking what is the value of something. I have partially dealt with this element earlier. The ultimate answer to the 'So What?!' type of question is the purpose of all existence. It is why we exist.
In searching for ever better explanations of how existence behaves we always expect a deeper level of description. Whatever answer I give to describe some aspect of reality, it is always rational to ask why in the sense of how. For example, Why is this piece of paper white? - Because the molecules in it scatter the light. Why? Because the chemicals in the paper reflect all wavelengths of light so that on average all wavelengths combine to make the reflected light white. Why do the molecules reflect light?.... and on and on and on. There are still many, many unanswered questions in science.
The point at which our reasoning comes to a stop is where our knowledge ends. Asking for explanations beyond that must yield the answer "I don't know". However, you may still theorise and ask 'what if ...' type questions. The point at which this questioning could end would be the point at which the concepts are beyond human understanding; when the explanation lies outside of human experience and therefore is in essence inexpressible in human language. This is the thing of which we cannot rationally ask 'how?'; it is that which, by its nature, we have to say we can't know.
This point in our explanations is the ultimate explanation of reality. It is the ultimate metaphysical reality.
You may have other names for it but it is the same thing - the truth - the beginning and end of everything - God - Jehovah- Allah.