In Islamic theology, the concept of parallel universes is not explicitly addressed in the Quran or Hadith. As a result, there is no specific ruling or consensus on whether believing in parallel universes is considered halal (permissible) or haram (forbidden) in Islam.
The understanding of the physical universe and its nature is a topic that falls under the realm of scientific exploration and discovery. Islam encourages the pursuit of knowledge and the study of the natural world. As long as the belief in parallel universes does not contradict any established Islamic teachings or principles, it can be considered a matter of personal belief or a scientific hypothesis.
It is important to note that Islamic beliefs should be grounded in the teachings of the Quran and the Sunnah (the practices and sayings of the Prophet Muhammad, peace be upon him). If a belief in parallel universes leads to beliefs or actions that are contrary to Islamic teachings, such as denying the oneness of Allah (Tawheed) or challenging the divine wisdom and purpose of creation, it may be considered problematic.
Ultimately, whether the belief in parallel universes is considered halal or haram would depend on how it aligns with Islamic teachings and whether it conflicts with core principles of the faith. Consulting with knowledgeable scholars or seeking guidance from Islamic sources can provide a more nuanced understanding of this matter.