The temporary decaying material creation of Maha-Visnu takes up 1/4 of the Spiritual Sky (all that exists)

All the material universes (Brahmandas) originate from the Body of Maha-Visnu via the pores of His skin and His breathing.

The other 3/4s of the Spiritual Sky are the perpetual "forever youthful and fresh" Vaikuntha planets where Viṣṇu/Narayana eternally reside, and Krsna's massive central planet called Goloka-Vrindavana. 

The material universes or individual Brahmandas, are an unlimited number of universes that take up 1/4 of the Spiritual Sky. 

All Brahmandas come from the Body Maha-Visnu and make up the vast material universe known as the mahat-tattva. 

Furthermore, each massive Brahmanda universe encases a smaller secondary universe within it where Garbhodakashayi Visnu and Brahma resides. 

There are billions of Garbhodakashayi Visnus and Lord Brahmas, but there is only one Maha-Visnu in the material creation.

Brahma goes on to create or build His many planetary systems within his particular universe.

So deep inside all the billions of Brahmanda universes, is a secondary material universe where Garbhodakashayi Visnu, Lord Brahma and His various demigods (administrators) reside. 

Our Lord Brahma has 4 heads making our inner material universe very small at only 4 billion miles in diameter. 

Other Lord Brahmas in other  universes inside their Brahmanda have 10 or 100 or 1000, 10,000, 1000,000 or 1000,000 heads or more!

This is inconceivable to our small brains.

This means the Brahmanda universe encasing our inner universe is very small compared to other Brahmanda universes.

There are billions of Garbhodakashayi Visnus and Brahmas and demigods but only one Maha-Visnu.

For some scholars and devotees, our "secondary material universe" inside our surrounding Brahmanda, almost matches the size of Bhu-Mandala, which is described as being close to 4 billion miles in diameter. (5th Canto of Srimad Bhagavatam)

For example, our material universe takes up the area Bhu-Mandala covers which Bhagavatam says is almost 4 billion miles in diameter. 

It is also explained in Srimad Bhagavatam that the secondary material universe inside our Brahmanda is one of the smallest, being only 4 billion miles (500,000,000 Yojanas) in diameter. 

One yojana equals 8 miles, so 500 million yojanas multiplied by 8 equals 4 billion miles.

There are 14 planetary systems in our 4 billion mile diameter universe, but the amount of planetary systems are different in other universes because they are much larger.

Within the stem of the lotus flower coming from the naval of Garbhodakashayi Visnu, there are fourteen divisions of planetary systems created by Lord Brahma, and our earth spherical global planet is situated in the middle of the stem.

Srila Prabhupada - "We have to imagine that these universes (Brahmandas), which according to our limited knowledge are expanded unlimitedly, are so great that the gross and subtle ingredients—the five elements of the cosmic manifestation, namely 

1 - Earth, 

2 - Water, 

3 - Fire, 

4 - Air,

5 - Sky, 

6 - Total material energy, 

7 - False ego.

All are not only part of the material universe but also encase our material universe with seven material layers of material qualities. Each layer is ten times bigger than the previous one. 

In this way, each and every material universe is very securely packed, and there are numberless Brahmanda universes and their inner secondary universes. All the Brahmanda universes float within the innumerable pores of the transcendental body of Mahā-Viṣṇu. 

It is stated that just as the atoms and particles of dust are floating within the air along with the birds and their number cannot be calculated, so innumerable universes are floating within the pores of the transcendental body of the Lord. For this reason, the Vedas say that God is beyond the grasp of our knowledge." (Adapted from Krsna Book 87)

We also cannot see from one universe to another.

This is because our individual universe is surrounded and encased by 7 thick material layers that makes up our outer Brahmanda universe.

It is impossible to see the millions of other Brahmandas outside our inner universe (we are inside a massive Brahmanda) because of being surrounded by 7 thick material layers.

However there are rare mystic yogis and advanced devotees like Arjuna who can see though these seven massive layers and pass through. 

The distance from the outer edge of our inner material universe to the external of the surrounding Brahmanda is 44 quadrillion, 444 trillion, 444 billion miles in diameter, or twice that size if the entire diameter of the Brahmanda is measured.

So of course Krsna and Arjuna did not encounter such limitations we experience by the combination of these elements like earth, water, fire, air all gross elements as Bhagavad Gita As It Is explains.

Srila Prabhupada - "The Lord is not visible to the eyes of ordinary men, those who are beyond the covering layers because of their transcendental devotional service can still see Him." (Krsna Book 87)

As said above, we cannot see through our Brahmanda's 7 layers of material energy that surrounds and encases our small secondary material universe within it with our telescopes and radio telescopes.

Srila Prabhupada explains further - "Lord Krsna and Arjuna crossed over the great region of darkness covering the material universes. Arjuna then saw the effulgence of light known as the brahmajyoti. The brahmajyoti is situated outside the covering of the material universes, and because it cannot be seen with our present eyes, this brahmajyoti is sometimes called avyakta." (Krsna Book, 89)

Srila Prabhupada - "Each universe is covered by various layers of material elements (The 7 layers of the outer Brahmanda covering), and therefore although the universes are clustered together, we cannot see from one universe to another. In other words, whatever we see is within this one universe. In each universe there is one Lord Brahmā, and there are other demigods on other planets, but there is only one sun." (SB Canto 5 Ch 21 Text 11, Purport)

Srila Prabhupāda - "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]. This verse describes the coverings of the universe (saptabhir daśa-guṇottarair aṇḍa-kośaḥ)." (SB Canto 6, Ch 16, Text 37) 

As said above there are 7 material layers of the greater Brahmanda universe that encase our 4 billion diameter inner secondary material universe.

Each layer is 10 times thicker than the previous beginning with multiplying the 4 billion miles diameter of our inner universe by 10 to get the diameter of the first layer which is earth- 

1 - Earth - 40 billion miles in diameter, 

2 - Water - 400 billion miles in diameter, 

3 - Fire - 4000 billion (4 trillion) miles in diameter, 

4 - Air - 40,000 billion (40 trillion) miles in diameter,

5 - Sky - 400,000 billion (400 trillion) miles in diameter,

6 - Total material energy - 4000,000 billion, (4 quadrillion) miles in diameter.

7 - False ego - 40,000,000 billion (40 quadrillion) miles in diameter. 

The total size of our Brahmanda that our small 4 billion mile diameter secondary universe is in, is 44 quadrillion 444 trillion 444 billion miles in diameter according to calculations from Canto 5 of Srimad Bhagavatam or possibly even twice that size.

Beginning with the covering of earth, each covering is ten times greater than the previous one. Thus we can only imagine how great and massive each Brahmanda outer universe is, and there are many billions of universes. 

Lord Krsna in Bhagavad-gītā As It Is says (BG 10.42)-

athavā bahunaitena

kiṁ jñātena tavārjuna

viṣṭabhyāham idaṁ kṛtsnam

ekāṁśena sthito jagat

"But what need is there, Arjuna, for all this detailed knowledge? With a single fragment of Myself I pervade and support this entire universe." 

The entire material world manifests only one fourth of the Supreme Lord's energy. Therefore He is called ananta." 

Srila Prabhupada  - "The first puruṣa is the Kāraṇodakaśāyī Viṣṇu. From His skin holes innumerable universes have sprung up. In each and every universe, the puruṣa enters as the Garbhodakaśāyī Viṣṇu.

He is lying within the half of the universe which is full with the water of His body. 

And from the navel of Garbhodakaśāyī Viṣṇu has sprung the stem of the lotus flower, the birthplace of Brahmā, who is the father of all living beings and the master of all the demigod engineers engaged in the perfect design and working of the universal order.

Within the stem of the lotus there are fourteen divisions of planetary systems, and the earthly planets are situated in the middle.

Upwards there are other, better planetary systems, and the topmost system is called Brahmaloka or Satyaloka. Downwards from the earthly planetary system there are seven lower planetary systems inhabited by the asuras and similar other materialistic living being." (SB Canto 1 Ch 3 text 2, Purport)

As said above our secondary material universe deep inside our Brahmanda is divided into fourteen planetary systems. 

Seven upper planetary systems- 

Bhūr,

Bhuvar, 

Svar, 

Mahar, 

Janas, 

Tapas, 

Satya, 

These are all upward planetary systems, one above the other. There are also seven planetary systems downward, known as-

Atala,

Vitala,

Sutala,

Talātala,

Mahātala,

Rasātala,

Pātāla.

From Matchless Wondergifts.