There is a Shloka in Shreemad Bhagvat Purana mentioning the concept of Multiverse OR Parallel Universes. – Skanda 6 –Chapter 16 –shloka 37

क्षित्यादिभिरेष किलावृतः सप्तभिर्दशगुणोत्तरैरण्डकोशः ।
यत्र पतत्यणुकल्पः सहाण्डकोटिकोटिभिस्तदनन्तः ॥

kṣity-ādibhir eṣa kilāvṛtaḥ saptabhir daśa-guṇottarair aṇḍa-kośaḥ
yatra pataty aṇu-kalpaḥ sahāṇḍa-koṭi-koṭibhis tad anantaḥ

" Every universe is covered by seven layers — earth, water, fire, air, sky, the total energy and false ego — each ten times greater than the previous one. There are innumerable universes besides this one, and although they are unlimitedly large, they move about like atoms in You. Therefore You are called unlimited [ananta] "

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Another shloka of Bhagvat Puraan giving an idea of multiple universe existence - Skanda 11 - Chapter 16 - shloka 39

संख्यानं परमाणूनां कालेन क्रियते मया। 
 न तथा मे विभूतीनां सृजतोऽण्डानि कोटिशः॥

From here on I am giving the Skanda, chapter and shloka number with there meaning so please refer from there - 

Even though over a period of time I might count all the atoms of the universe, I could not count all of My opulences which I manifest within innumerable universes (Srimad Bhagavatam 11.16.39)

What am I, a small creature measuring seven spans of my own hand? I am enclosed in a potlike universe composed of material nature, the total material energy, false ego, ether, air, water and earth. And what is Your glory? Unlimited universes pass through the pores of Your body just as particles of dust pass through the openings of a screened window (Srimad Bhagavatam 10.14.11)

The layers or elements covering the universes are each ten times thicker than the one before, and all the universes clustered together appear like atoms in a huge combination (Srimad Bhagavatam 3.11.41)

A mention of parallel universe in Brahma Vaivarta Puran

And who will search through the wide infinities of space to count the universes side by side, each containing its Brahma, its Vishnu, its Shiva? Who can count the Indras in them all–those Indras side by side, who reign at once in all the innumerable worlds; those others who passed away before them; or even the Indras who succeed each other in any given line, ascending to godly kingship, one by one, and, one by one, passing away?

            There are innumerable mentions of multiple universes in our ancient scriptures. All we need is to read and explore them.

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Answer of the previous question - Whenever we offer anything in the hawan kund, it is recommended to recite 'Swaha'. It is said that Swaha, daughter of King Daksha, is the wife of Agnidev. So if one wants Agnidev to accept his/her offerings, it is a must to remember his wife with it otherwise none will be accepted and the havan will not be completed.

Another story as per the folklores - Shri Krishna has blessed Swaha that none of the offering can be accepted by the deities without offering it to her.

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Question of the day - The place where it is believed that Mahadev and Mata Parvati married ?